IrcUser: Use QDateTime for lastAwayMessage, not int

This converts the lastAwayTime to a 64-bit time.  Backwards-
compatibility is also included in Network to convert back and forth
between the old 32-bit type for legacy peers.  No backwards-
compatibility is provided for the SYNC call because the value is
never actually used in any existing clients.  New clients that want
to implement this functionality would need to implement the
LongTime feature.