THRIFT-926. cpp: Fix bugs in TFileTransport::flush()

THRIFT-926. cpp: Fix bugs in TFileTransport::flush()

Previously flush() had race conditions that could cause it to return
before all data had actually been flushed to disk.  Now the writer
makes sure both buffer queues have been flushed when forceFlush_ is set.

Also, flush() did not wake up the writer thread, so it normally had to
wait for the writer thread to wake up on its own time.  (By default,
this could take up to 3 seconds.)

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@1005156 13f79535-47bb-0310-9956-ffa450edef68