Merge from #7596:
 * r43054 (copy old time to new log_time)
 * r43088, r43090 and r43095 (tests for timestamp in output in old logging system)
 * r43091 (LegacyLogObserverWrapper should provide time and system)
 * r43092 (restores timestamps)


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/log-booyah-6750-5@43464 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb