mon/OSDMonitor: record pre-octopus purged snaps with first octopus map

mon/OSDMonitor: record pre-octopus purged snaps with first octopus map

When we public our first require_osd_release >= octopus osdmap, record
all prior purged snaps in a key linked to the previous osdmap.  We assume
this will encode and fit into a single key and transaction because the
even larger set of removed_snaps is already a member of pg_pool_t, which
is included in every osdmap.

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