PGBackend: add interfaces for stashing removed objects

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: ObjectModDesc

Contains information needed to roll back a pg_log_entry_t.

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: add ObjectModDesc into pg_log_entry_t

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: add a can_rollback_to line to pg_log_t

This allows us to determine how far back the log can be rolled.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend,ReplicatedBackend: add support for rolling back log entries

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: introduce xattr caching for ec pools

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: Use RollBacker interface

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: Fill in rollback info for log events

Signed-off-by: Samuel Just <sam.just@inktank.com>