osd/osd_types: pg_pool_t: add FLAG_{SELFMANAGED,POOL}_SNAPS flags

osd/osd_types: pg_pool_t: add FLAG_{SELFMANAGED,POOL}_SNAPS flags

Explicitly track whether we are a pool snaps pool or a selfmanaged
snaps pool.  This was inferred from removed_snaps.empty() before, but
that was fragile and kludgey and removed_snaps is going away.

The upgrade/compat behavior is a bit tricky:

- on decode, we set the flags based on the legacy condition.  This lets us
use and rely on the flags in memory.
- on encode, we exclude the flags if decoding an older pg_pool_t

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