[LLGS] Get rid of the stdio forwarding thread
This commit removes the stdio forwarding thread in lldb-server in favor of a MainLoop callback.
As in some situations we need to forcibly flush the stream ( => Read() is called from multiple
places) and we still have multiple threads, I have had to additionally protect the communication
instance with a mutex.
Reviewers: ovyalov, tberghammer
Differential Revision: http://reviews.llvm.org/D11296