This change set addresses a flake in TestSendBigMessage that happens
due to creating the stream too early and freezing too late.

Now the code first freezes, and only then creates the stream.

Also switched the waitGroup to a condition variable which is more
idiomatic for this use case.

Change-Id: I673e8d6cc38caf68fd298dc1a3acfc0af1302961
Signed-off-by: yacovm <yacovm@il.ibm.com>