Add TimedWait() API to CondVar.

Summary:
Add TimedWait() API to CondVar, which will be used in the future to
support TimedOut Write API and Rate limiter.

Test Plan: make db_test -j32

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19431