[LLGS] Get rid of the stdio forwarding thread

Summary:
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

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11296

llvm-svn: 242782