mon/OSDMonitor: set last_force_resend_prenautilus for pg_num_pending changes

mon/OSDMonitor: set last_force_resend_prenautilus for pg_num_pending changes

This will force pre-nautilus clients to resend ops when we are adjusting
pg_num_pending.  This is a big hammer: for nautilus+ clients, we only have
an interval change for the affected PGs (the two PGs that are about to
merge), whereas this compat hack will do an op resend for the whole pool.
However, it is better than requiring all clients be upgraded to nautilus in
order to do PG merges.

Note that we already do the same thing for pre-luminous clients both for
splits, so we've already inflicted similar pain the past (and, to my
knowledge, have not seen any negative feedback or fallout from that).

Signed-off-by: Sage Weil <sage@redhat.com>