osd: fix off-by-one in map gap logic

If we have map 250, and monitor's first is 251, but sends 260, we can
request the intervening range.

Fixes: #5784
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: get initial full map after a map gap

If there is a gap in our map history, get the full range of maps that
the mon has.  Make sure the first one is a full map.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>