THRIFT ReadWriteMutex

THRIFT ReadWriteMutex

Summary: Implemented a read/write lock class in thrift::concurrency

Reviewed By: marc

Test Plan: I compiled this, then compiled a version of fb303 that uses this
(future checkin pending further tests), then ran a binary using the new fb303
and it didn't crash as things were added to the map using the ReadWriteMutex

Revert Plan: yup

Notes: this should replace rw_lock.h for most clients

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665154 13f79535-47bb-0310-9956-ffa450edef68