Move GetThreadList() feature under Env.

Summary:
GetThreadList() feature depends on the thread creation and destruction, which is currently handled under Env.
This patch moves GetThreadList() feature under Env to better manage the dependency of GetThreadList() feature
on thread creation and destruction.

Renamed ThreadStatusImpl to ThreadStatusUpdater.  Add ThreadStatusUtil, which is a static class contains
utility functions for ThreadStatusUpdater.

Test Plan: run db_test, thread_list_test and db_bench and verify the life cycle of Env and ThreadStatusUpdater is properly managed.

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: ljin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30057