Summary:
Add Env::GetThreadID(), which returns the ID of the current thread.

In addition, make GetThreadList() and InfoLog use same unique ID for the same thread.

Test Plan:
db_test
listener_test

Reviewers: igor, rven, IslamAbdelRahman, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D39735