Also ensure github-linguist does not parse this file as Raku.

Signed-off-by: Thibault Charbonnier <thibaultcha@me.com>

Specifying a `nil` callback to `:get()` is now possible. Doing so will
return any item from L1/L2 (and do the L2 -> L1 promotion as usual).

If a requested key is not in the cache, the return values will be `nil,
nil, -1`, where -1 is a new meaning for the `hit_lvl` return value
indicating a cache miss. This is helpful when trying to distinguish a
*negative hit* (cached `nil`) from a regular *miss* (no value). The
former will have a positive `hit_lvl` indicating the negative hit was
cached in L1/L2).

Example:

    local val, err, hit_lvl = cache:get("key")
    if val == nil then
        if hit_lvl == -1 then
            -- miss (no value)
        end

        -- negative hit (cached `nil`)
    end

Thanks Hamish Forbes and Corina Purcarea for the feature proposal and
patch.

From #96

lua-resty-lru's `get()` now returns a third value `flags` in 1.17.8.x.

Fix #93

Fix #93

This test should have been enabled a long time ago already.

…tems

Fix #66

A follow-up to #88.

Described in #93 and reported a few months ago is the likely possibility
that mlcache and ipc instances are created in the `init` phase. If a
worker is to be started much later in the master process' lifetime, the
newly forked ipc instance will have an `idx` attribute set to the
shm_idx value at the time of `init` (likely 0). These workers will
resume polling evicted events, and `poll()` will likely timeout
indefinitely from there.

For the fix in #88 to work, the mlcache instance has to be instantiated
during `init_worker` or later.

This patch proposes an approach which works for instances created in
both `init` and `init_worker`: as soon as the ipc shm has started
evicting items, we guarantee that future workers will resume polling at
the current index, without having to call any method but `poll()`.

Fix #93