More bullet proofing of timer manager

More bullet proofing of timer manager

facebook::thrift::concurrency::TimerManager::stop
	Added proper cleanup of unprocessed tasks and shutdown of dispatcher thread to stop

facebook::thrift::concurrency::TimerManager::~TimerManager
	Call stop if manager wasn't explicitly stopped

facebook::thrift::concurrency::test.TimerManagerTest
	Calculate error margin for timeout expiration and verify it's within bounds
	Verify manager stops properly when it goes out of scope

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@664724 13f79535-47bb-0310-9956-ffa450edef68