osd/: refactor PGLog a bit and add support for rolling back extents

osd/: refactor PGLog a bit and add support for rolling back extents

It was hard to reason about the validity of the IndexedLog internal
pointers and iterators during updates, so this patch cleans that up
a bunch.  It also moves responsibility for doing rollbacks into
PGBackend.  Finally, it adds support for the new log entry format.

Signed-off-by: Samuel Just <sjust@redhat.com>