[0.1.x] add set_default to 0.1 executor, timer, and reactor

[hawkw]

This commit adds set_default drop guard style APIs for setting the
default reactor, executor, and timer. These are similar to the APIs used
in tokio 0.2

In addition to having potentially better ergonomics than the
with_default closure APIs, the drop-guard based APIs will be helpful
in rewriting the tokio-compat crate to wrap the existing tokio 0.2
runtime, rather than constructing its own runtime.

Because the runtime does not expose an around_worker API, it cannot
currently be used with the 0.1 with_default method of setting the
reactor, timer, and executor. This means that tokio-compat must
duplicate a lot of existing code from tokio to construct the runtime,
which is unfortunate (and has the potential to introduce errors). On the
other hand, we can use the drop guard APIs with before_start and
after_stop, by storing the drop guards in a thread-local. This will
allow tokio-compat to wrap the 0.2 runtime, reducing code duplication.
Also, this will allow the blocking pool to be used on the compat
runtime, which is currently impossible (as the blocking APIs are private
to tokio).

Signed-off-by: Eliza Weisman eliza@buoyant.io

[hawkw]

The "Test async-await" CI failure seems unrelated?

[GeorgeHahn]

Could set_default be implemented for Clock as well?

I'm excited to see this PR! I have been thinking about building something like this to improve our libevent-tokio integration. This new API will allow us to have tokio defaults set when we are executing outside of a tokio context.

(For context, we have a very large amount of C that runs outside of a tokio runtime context. This C code may call into Rust via FFI or callbacks. We must avoid calling tokio functions in these cases because the missing defaults will cause a panic.)

[hawkw]

@GeorgeHahn Yup, added clock::set_default as well, thanks for pointing that out!

[hawkw]

PR #1742 will fix the broken CI builds for this branch, which are unrelated to the rest of this change. When that branch merges, I'll rebase this one and merge it, unless there are any objections?