Upgrade TTLCache to 1.5.0

Merge pull request #9 from Appboy/ttlcache-1.5.0

Upgrade ttlcache to 1.5.0

Upgrade TTLCache to 1.4.0

Fixes data race, reduces memory usage

Upgrade TTLCache

Merge pull request #7 from Appboy/upgrade-ttlcache

I need this newer version in a dependency

0.0.2

Mostly extremely minor documentation polish, and this change:

When the pool is fully utilized, buffer additional work until we have an entire pool size worth of enqueued work, to reduce blocking

Initial Release

Merge pull request #2 from Appboy/initial-import

Add Readme to worker-pools