Simplified message processing and semantics.

In _tcp_task, process replies in one place instead of in each
case for different types of messages.

'deliver' method is synchronous, so use _sync_reply.

Synchronous messages don't (need to) set 'src' attribute.
'dst' attribute should be set only when message is being delivered
to specific location (so _async_reply takes 'dst' as an argument
in case message is being delivered to all/any peer).