[ORC] Make sure RPC channel-send is called in blocking calls and responses.

ORC-RPC batches calls by default, and the channel's send method must be called
to transfer any buffered calls to the remote. The call to send was missing on
responses and blocking calls in the SingleThreadedRPCEndpoint. This patch adds
the necessary calls and modifies the RPC unit test to check for them.

llvm-svn: 371245