Graceful termination

[karalabe]

Implement graceful termination to flush all buffered messages before closing the relay link. Will probably need some relay side fixes too not to accept inbound messages after termination has been initiated.

[karalabe]

This needs revisiting since a lot of changes went into the relay both binding as well as relay side.

Also, this issue would seriously benefit some tests, as without them it's hard to argue whether a problem exists and whether a change fixes it.