add panic handler to async client/channel writes

See #2113 for motivation

irc/channel.go

 
 // equivalent of Socket.send()
 func (channel *Channel) writeLoop() {
+	defer channel.server.HandlePanic()
+
 	for {
 		// TODO(#357) check the error value of this and implement timed backoff
 		channel.performWrite(0)

irc/client.go

 }
 
 func (client *Client) writeLoop() {
+	defer client.server.HandlePanic()
+
 	for {
 		client.performWrite(0)
 		client.writebackLock.Unlock()