PGLog: add rollback_info_trimmed_to_riter to IndexedLog and use in PG

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: plumb rollback_trim_to through interfaces and messages

We simply trim the roll back info up to min_last_complete_ondisk
since we cannot correctly rollback past that point anyway!

ReplicatedPG
-> PGBackend::submit_transaction
-> Backend messages
-> PGBackend::Listener::log_operation
-> append_log

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: in scrub, check for and remove obsolete rollback objs

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