emacs: notmuch-lib: add refresh all buffers function

notmuch-refresh-all-buffers calls each buffer's major mode specific
refresh function using the generic notmuch-refresh-this-buffer function.

Since the earlier changesets have stopped the refresh functions from
forcing the buffers to be redisplayed this can refresh buffers that
are not currently displayed without disturbing the user.  This is very
useful for silent async background updating the emacs display when new
mail is fetched.

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>