don't attempt to start a rekey negotiation from within send_message -- always do it from the feeder thread. this prevents a situation where more than one thread may decide spontaneously to rekey, sending multiple kexinit messages, which confuses the hell out of the remote host :) also, do some locking around the clear-to-send event, to avoid a race when we first go into rekeying. add some tests for these things too
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