test: add parallel clean_pg_upmaps test

test: add parallel clean_pg_upmaps test

With parallel clean_pg_upmaps feature on, the total time cost
of the performance test which now can utilize up to 8 threads for
parallel upmap validating decreased from:

maybe_remove_pg_upmaps (~10000 pg_upmap_items) latency:104s

to:

clean_pg_upmaps (~10000 pg_upmap_items) latency:7s

Note that by default the mon uses only 4 worker threads for
CPU intensive background work, you could further increase
the "mon_cpu_threads" option value if you decided the
time-consuming of clean_pg_upmaps still matters.

Fixes: http://tracker.ceph.com/issues/40104
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>