crimson/osd: introduce RollbackOrchestrator to OpsExecuter.

crimson/osd: introduce RollbackOrchestrator to OpsExecuter.

If the execution of an `OSDOp` fails, we're left with potentially
altered `ObjectContext`. We deal with that by reloading `obc` if
there was any modification to it. To figure this out, `has_seen_write()`
on `OpsExecuter` is being called. Unfortunately, the current impl.
has following drawbacks:

 * `has_seen_write()` can be called after `std::move(ox).flush_...()`
    which is very inelegant;
 * it requires catching both `ObjectContext` and `OpsExecuter` while
   the latter already references the former;
 * there is no explicitly given reason in the header for justifying
   the presence of `has_seen_writes()`.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>