osd: Add snaptrim duration to pg dump stats.

osd: Add snaptrim duration to pg dump stats.

Add the snaptrim duration to the json formatted output of the pg dump
stats. Define methods for a PG to set the snaptrim begin time and then to
calculate the total time spent to trim all the objects for the snaps in
the snap_trimq for the PG.

Tests:
  - Librados C and C++ API tests to verify the time spent for a snaptrim
    operation on a PG. These tests use the self-managed snaps APIs.
  - Standalone tests to verify snaptrim duration using rados pool snaps.

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