mon, osd: Add objects trimmed to pg dump stats.

mon, osd: Add objects trimmed to pg dump stats.

Add a new column, OBJECTS_TRIMMED, to the pg dump stats that shows the
number of objects trimmed when a snap is removed.

When a pg splits, the stats from the parent pg is copied to the child
pg. In such a case, reset objects_trimmed to 0 for the child pg
(see PeeringState::split_into()). Otherwise, this will result in incorrect
stats to be shown for a child pg after the split operation.

Tests:
 - Librados C and C++ API tests to verify the number of objects trimmed
   during snaptrim operation. These tests use the self-managed snaps APIs.
 - Standalone tests to verify objects trimmed using rados pool snaps.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>