ThreadPool to allow decrease number of threads and increase of number of threads is to be instantly scheduled

Summary:
Add a feature to decrease the number of threads in thread pool.
Also instantly schedule more threads if number of threads is increased.

Here is the way it is implemented: each background thread needs its thread ID. After decreasing number of threads, all threads are woken up. The thread with the largest thread ID will terminate. If there are more threads to terminate, the thread will wake up all threads again.

Another change is made so that when number of threads is increased, more threads are created and all previous excessive threads are woken up to do the work.

Test Plan: Add a unit test.

Reviewers: haobo, dhruba

Reviewed By: haobo

CC: yhchiang, igor, nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D18675
Print out thread ID while thread terminates for decreased pool size.

Summary: Per request from @nkg-, temporarily print thread ID when a thread terminates. It is a temp solution as we try to minimized stderr messages.

Test Plan: env_test

Reviewers: haobo, igor, dhruba

Reviewed By: igor

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D18753