Raw connections do not always complete close operations

The Proton raw_connection_t currently requires cooperation from the application layer to complete a close.  There is a baked in assumption that the application will always eventually provide a read buffer.  A second assumption is that the peer (not necessarily a Proton raw connection) will detect a read close on its side, and do a graceful close of it's write side "soon".

These incorrect assumptions can leave the raw connection in a hung state waiting for non-existent wind up activity by the application or peer, respectively.