osd: improve debug output on snap collections

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: unconditionally encode snaps buffer

Previously we would only encode the updated snaps vector for CLONE ops.
This doesn't work for MODIFY ops generated by the snap trimmer, which
may also adjust the clone collections.  It is also possible that other
operations may need to populate this field in the future (e.g.,
LOST_REVERT may, although it currently does not).

Fixes: #4071, and possibly #4051.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: include snaps in pg_log_entry_t::dump()

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: update snap collections for sub_op_modify log records conditionaly

The only remaining caller is sub_op_modify().  If we do have a non-empty
op transaction, we want to do this update, regardless of what we think
last_backfill is (our notion may be not completely in sync with the
primary).  In particular, our last_backfill may be the same object but
a different snapid, but the primary disagrees and is pushing an op
transaction through.

Instead, update the collections if we have a non-empty transaction.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>