Bundle a `uv_async_t`, a `uv_idle_t`, a `uv_mutex_t`, a `uv_cond_t`,
and a `v8::Persistent<v8::Function>` to make it possible to call into JS
from another thread. The API accepts a void data pointer and a callback
which will be invoked on the loop thread and which will receive the
`napi_value` representing the JavaScript function to call so as to
perform the call into JS. The callback is run inside a
`node::CallbackScope`.

A `std::queue<void*>` is used to store calls from the secondary
threads, and an idle loop is started by the `uv_async_t` callback on the
loop thread to drain the queue, calling into JS with each item.

Items can be added to the queue blockingly or non-blockingly.

The thread-safe function can be referenced or unreferenced, with the
same semantics as libuv handles.

Re: nodejs/help#1035
Re: nodejs#20964
Fixes: nodejs#13512
PR-URL: nodejs#17887
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

private field 'async_context' is not used [-Wunused-private-field]

PR-URL: nodejs#21597
Refs: nodejs#17887
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

A condition variable is only created by the thread-safe function if the
queue size is set to something larger than zero. This adds null-checks
around the condition variable and tests for the case where the queue
size is zero.

Fixes: nodejs/help#1387
PR-URL: nodejs#21871
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

PR-URL: nodejs#21898
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

The idle_running member variable in TsFn is always false and can
therefore be removed.

PR-URL: nodejs#22520
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Gus Caplan <me@gus.host>

* Move class `TsFn` to name space `v8impl` and rename it to
  `ThreadSafeFunction`
* Remove `NAPI_EXTERN` from API declarations, because it's only needed
  in the header file.

PR-URL: nodejs#22259
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

The thread_finalize_data and thread_finalize_cb parameters in
napi_create_threadsafe_function are optional.

PR-URL: nodejs#22998
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Currently when building with --debug
test/addons-napi/test_threadsafe_function will error:

$  out/Debug/node test/addons-napi/test_threadsafe_function/test.js
FATAL ERROR: v8::HandleScope::CreateHandle()
  Cannot create a handle without a HandleScope
 1: 0x10004e287 node::DumpBacktrace(__sFILE*) [node/out/Debug/node]
 2: 0x1000cd37b node::Abort() [/node/out/Debug/node]
 3: 0x1000cd69f node::OnFatalError(char const*, char const*)
    [/node/out/Debug/node]
 4: 0x1004df0b1 v8::Utils::ReportApiFailure(char const*, char const*)
    [/nodejs/node/out/Debug/node]
 5: 0x100a8c0a9 v8::internal::HandleScope::Extend(
        v8::internal::Isolate*)
    [/node/out/Debug/node]
 6: 0x1004e4229 v8::EmbedderDataFor(v8::Context*,
                                    int, bool,
                                    char const*)
    [/node/out/Debug/node]
 7: 0x1004e43fa v8::Context::SlowGetAlignedPointerFromEmbedderData(int)
    [/node/out/Debug/node]
 8: 0x10001c26b v8::Context::GetAlignedPointerFromEmbedderData(int)
    [/node/out/Debug/node]
 9: 0x1000144ea node::Environment::GetCurrent(v8::Local<v8::Context>)
    [/node/out/Debug/node]
10: 0x1000f49e2 napi_env__::node_env() const
    [/node/out/Debug/node]
11: 0x1000f9885
    (anonymous namespace)::v8impl::ThreadSafeFunction::
        CloseHandlesAndMaybeDelete(bool)
    [/node/out/Debug/node]
12: 0x1000fb34f (anonymous namespace)::v8impl::ThreadSafeFunction::
        DispatchOne()
    [/node/out/Debug/node]
13: 0x1000fb129
    (anonymous namespace)::v8impl::ThreadSafeFunction::
        IdleCb(uv_idle_s*)
    [/node/out/Debug/node]
14: 0x1011a1b69 uv__run_idle
    [/node/out/Debug/node]
15: 0x101198179 uv_run
    [/node/out/Debug/node]
16: 0x1000dfca1
    node::Start(...)
    [/node/out/Debug/node]
17: 0x1000dae50 node::Start(...)
    [/node/out/Debug/node]
18: 0x1000da56f node::Start(int, char**)
    [/node/out/Debug/node]
19: 0x10141112e main
    [/node/out/Debug/node]
20: 0x100001034 start
    [/node/out/Debug/node]
Abort trap: 6

This commit adds two HandleScope's, one to CloseHandlesAndMaybeDelete
and one to the lambda.

SlowGetAlignedPointerFromEmbedderData will only be called for debug
builds:
https://github.com/v8/v8/blob/2ef0aa662fe907a1b36ac1abe7d77ad2bcd27733
/include/v8.h#L10440-L10447

PR-URL: nodejs#24011
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

The test fails consistently on windows-fanned with vs2017.
mark it as flaky while the issue is being progressed, and
to keep CI green / amber.

Ref: nodejs#23621
PR-URL: nodejs#24714
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

In 180f865, the test was changed
so that the `env` argument of `createInternalRepl()` also contained
external environment variables, because keeping them can be necessary
for spawning processes on some systems.

However, this test does not spawn new processes, and relies on the
fact that the environment variables it tests are not already set
(and fails otherwise); therefore, reverting to the original state
should fix this.

Fixes: nodejs#21451
Fixes: nodejs/build#1377
Refs: nodejs#25219

PR-URL: nodejs#25226
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>