Introduce ThreadDeathWatcher

Motivation:

PooledByteBufAllocator's thread local cache and
ReferenceCountUtil.releaseLater() are in need of the way to run an
arbitrary logic when a certain thread is terminated.

Modifications:

- Add ThreadDeathWatcher, which spawns a low-priority daemon thread
  that watches a list of threads periodically (every second) and
  invokes the specified tasks when the associated threads are not alive
  anymore
- Add debug-level log messages to see if ThreadDeathWatcher works

Result:

- Fixes #2519 because we don't use GlobalEventExecutor anymore
- Cleaner code

/cc @tea-dragon