mon/OSDMonitor: do clean_pg_upmaps the parallel way if necessary

mon/OSDMonitor: do clean_pg_upmaps the parallel way if necessary

There could definitely be some certain cases we could reliably
skip this kind of checking, but there is no easy way to separate
those out.
However, this is clearly the general way to do the massive pg
upmap clean-up job more efficiently and hence should make sense
in all cases.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>