THRIFT-926. cpp: Make TZlibTransport::flush() behave like other transports

THRIFT-926. cpp: Make TZlibTransport::flush() behave like other transports

Previously, TZlibTransport::flush() finished the zlib stream, so calling
write() after flush() would result in an error.  Now it just flushes the
data, without finishing the stream.  A new TZlibTransport::finish()
function has been added to finish the stream.

This breaks compatibility.  I'm aware of anyone using this code outside
of Facebook, though.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@1005151 13f79535-47bb-0310-9956-ffa450edef68