librbd/object_map: allow intermediate snaps to be skipped on diff-iterate

librbd/object_map: allow intermediate snaps to be skipped on diff-iterate

In case of diff-iterate against the beginning of time, the result
depends only on the end version.  Loading and processing object maps
or intermediate snapshots is redundant and can be skipped.

This optimization is made possible by commit be507aaed15f ("librbd:
diff-iterate shouldn't ever report "new hole" against a hole") and, to
a lesser extent, the previous commit.

Getting FastDiffInvalid, LoadObjectMapError and ObjectMapTooSmall to
pass required tweaking not just expectations, but also start/end snap
ids and thus also the meaning of these tests.  This is addressed in the
next commit.

Fixes: https://tracker.ceph.com/issues/63341
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 23c675f04a4b4bb00794bbc59493d9591625a0a7)