Relay kill switch

[karalabe]

Currently almost everything can be tested easily in binding implementations, except proper clean-up in case of an Iris node failure. This would need each such test to kill off the Iris node after setting up the test, which would be very impractical.

A better solution would be to introduce a small kill switch into the relay protocol, through which each connection could trigger an immediate drop from the Iris node. This way we could simulate an Iris crash without needing to go through the hassle of actually producing one.