PGLog: initialize complete_to

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: fix logging in trim

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: add rollback_info_trimme_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>
PG: block outgoing peering messages until the last interval is durable

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-7999', 'origin/wip-8396', 'origin/wip-8438' and 'upstream/next' into wip-sam-testing