Fix missing flush in StreamCopyThread

It is possible to miss flush() invocation in StreamCopyThread.
In this case some data will not be sent to remote host and we will
wait forever (or until timeout) in src.read().

Use a counter to keep track of the flush requests.

Change-Id: Ia818be9b109a1674d9e2a9c78e125ab248cfb75b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>