osd/SnapMapper: maintain the prefix_itr between calls to SnapMapper::get_next_objects...

osd/SnapMapper: maintain the prefix_itr between calls to SnapMapper::get_next_objects_to_trim()
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

Disabled the assert as there is a corner case when we retrieve the last valid object/s in a snap
The prefix_itr is advanced past the last valid value (as we completed a full scan)
If the OSD will call get_next_objects_to_trim() before the retrieved object/s was processed and removed from the SnapMapper DB it won't be found by the next call (as the prefix_itr is invalid).
The object will be found in the second-pass which will seems as if it was added after the trim was started (which is illegal) and will trigger an ASSERT

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