znc playback timestamp should expire

[slingamn]

Right now, the timestamps from a PRIVMSG to *playback get stored in the Session object, and are used for replay on each JOIN. This is unavoidable because (for example) Textual completes registration, sends the PRIVMSG, and then sends JOIN lines, so on a new connection (as opposed to a reattach), we need to hold the information and wait for subsequent JOINs.

However, we shouldn't hold the information indefinitely; it should expire after a fixed period (maybe 60 seconds) and not affect subsequent manual /join commands.