chatView: Improve handling of initial pending messages

When a room has pending messages when we join, we currently
simply insert them into the log. While simple, there are
two issues with that approach:

 - we will always display the nick for the first pending
   message, so grouping will be wrong if the last message
   from the logs was from the same nick
 - if the message was already seen by the logger, we will
   duplicate it in the chat log (e.g. when mission-control
   restarts us after a crash)

Address those issues by caching pending messages and only
inserting them with the first batch of (de-duplicated) logs
instead of immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=768907