Keep messages sorted by timestamp

This causes messages that appear on some servers and not others to be
correctly interleaved on initial load.

Note: this is incomplete. rcupdatemessagetime() can break history's
always-in-sorted-order invariant by changing Time after messages have
been inserted.  Fixing this is next.

Consider replacing rcaddmessagetohistory()'s simple backwards linear scan
with a binary search in the unlikely event that its performance is ever
noticed.