ReplicatedPG: Fill in rollback info for log events

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: create interface allowing interface user to cleanup/rollback

We need to be able to allow the PGLog interface user to provide
logic for rolling back and trimming log entries.  To that end,
serveral PGLog methods now take a LogEntryHander.

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/test/osd/TestPGLog: fix PGLog test cases for changed methods

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