PGLog: on split, leave log head alone

This way last_update doesn't go backwards.

Fixes: 6447
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 0c2769d3321bff6e85ec57c85a08ee0b8e751bcb)
PGLog: remove obsolete assert in merge_log

This assert assumes that if olog.head != log.head, olog contains
a log entry at log.head, which may not be true since pg splitting
might have left the log with arbitrary holes.

Related: 0c2769d3321bff6e85ec57c85a08ee0b8e751bcb
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 353813b2e1a98901b876790c7c531f8a202c661d)
Merge pull request #1559 from ceph/wip-7881-dumpling

Wip 7881 dumpling

Reviewed-by: Sage Weil <sage@inktank.com>