osd/PGLog: do not use unique_ptr explicitly

osd/PGLog: do not use unique_ptr explicitly

* do not use unique_ptr<> explicitly, use `seastar::do_with()` for
  better readability
* use seastar::do_until() instead of seastar::repeat() for better
  readability. plain boolean is simpler than
  `seastar::stop_iteration::yes`
* do not capture variables using FuturizedStoreLogReader if we could
  pass them by instant parameters.
* rename "start()" to "read()". as "read" is more specific in this
  context.

Signed-off-by: Kefu Chai <kchai@redhat.com>