updateTS: don't save any broken TS values lower than 750000

Workaround for #385, and other related timestamp issues caused by the TS value getting cut off in mode messages (#283)

(cherry picked from commit ba330bbfac)

classes.py

@@ -1295,6 +1295,9 @@ class Protocol():
                 _apply()
 
             elif (their_ts < our_ts):
+                if their_ts < 750000:
+                    log.warning('(%s) Possible desync? Not setting bogus TS %s on channel %s', self.irc.name, their_ts, channel)
+                else:
                     log.debug('(%s) Resetting channel TS of %s from %s to %s (remote has lower TS)',
                               self.irc.name, channel, our_ts, their_ts)
                     self.irc.channels[channel].ts = their_ts