unittest_osdmap aborted during OSDMapTest.BUG_42485

unittest_osdmap aborted during OSDMapTest.BUG_42485

The testcase is aborted during the call of clean_upmap_tp
thread. The function(clean_pg_upmaps) spawns a number
of worker threads to process a PGMapper job. The worker
thread fetch a job from the queue and then process the
job and call process_finish the job. The process function
of PGMapper class destroying the object and as worker thread
call _process_finish function it crashes because job pointer
has become a dangling pointer.

Solution: To avoid a crash destroy the object in  _process_finish
          instead of doing in _process.

Fixes: https://tracker.ceph.com/issues/62934
Signed-off-by: Mohit Agrawal <moagrawa@redhat.com>