client: Add AsNeededBacklogRequester, faster login

Add AsNeededBacklogRequester, which behaves exactly like the
FixedBacklogRequester for legacy cores older than v0.13.0
(lacking Feature::BufferActivitySync).  For modern cores, no backlog
is initially fetched.  This should speed up logging in with no loss of
buffer activity signaling.

This breaks the "Show messages from backlog" feature as no backlog is
fetched.  The original always-fetch backlog requesters remain
available for those who prefer having the backlog in the Chat Monitor.

Add a warning to the Chat Monitor settings page when
"Only fetch when needed" Backlog Fetching is enabled.

Don't allow BacklogSettings to return the GlobalUnread backlog
requester type.  The UI for choosing it has been disabled, but it
could have accidentally been selected in the past.  If this had
happened before this change, AsNeeded will be selected instead.