osd/SnapMapper:

osd/SnapMapper:
Maintain the prefix_itr between calls to SnapMapper::get_next_objects_to_trim() to prevent searching depleted prefixes.
We got 8 distinct hash prefixes used for searching objects owned by a given PG.
On each call to SnapMapper::get_next_objects_to_trim() we start from the first prefix even after all objects mapped to it were depleted.
This means that we will be searching for 1 non-existing prefix after the first prefix was depleted, 2 after the first two prefixes were depleted... and so on until we will search 7 non-existing prefixes after the first 7 prefixes were depleted.

This is a performance improvement PR only!
It maintains the existing behavior and does not try to fix/change any of the TRIM logic.
I added an extra step after the last object is trimmed doing a full scan of the DB and only if no object was found it will return ENOENT.
This should make the new code no-worse than existing code which returns ENOENT after a full scan found no object.
It should not impact performance in real life snaps as it should only happen once per-snap.

added snap-mapper tests to rados-test-suite
disabled osd_debug_trim_objects when running (SnapMapperTest, prefix_itr) to prevent asserts(as this code does illegal inserts into DELETED snaps)
Code beautifing

Signed-off-by: Gabriel BenHanokh <gbenhano@redhat.com>