Add ReadOrAgain and WriteOrAgain methods to FuseMessage.

These methods return kAgain if operation cannot be done without blocking
the current thread.

The CL also introduecs new helper function SetupMessageSockets so that
FuseMessages are always transfered via sockets that save message
boundaries.

Bug: 34903085
Test: libappfuse_test
Change-Id: I34544372cc1b0c7bc9622e581ae16c018a123fa9