emacs: When refreshing a show buffer, only mark read when resetting state

If we retain state while refreshing a show buffer, it should not mark
any messages read since it's not a navigation operation (it especially
shouldn't mark the first message matching the query read, which is
what it did previously).  If the user or caller requests that refresh
reset the state of the buffer, then we consider that a navigation
operation, so we do mark the message under point after the refresh
read.

This is implemented by moving responsibility for initial positioning
and read-marking out of notmuch-show-worker and into its caller.
Since notmuch-show-worker is now exclusively about building the show
buffer, we rename it to notmuch-show-build-buffer.