rgw/dbstore: Handle read vs delete races

rgw/dbstore: Handle read vs delete races

Now that tail objects are associated with objectID, they are not deleted
as part of this DeleteObj operation. Such tail objects (with no head object
in *.object.table are cleaned up later by GC thread.

To avoid races between writes/reads & GC delete, mtime is maintained for each
tail object. This mtime is updated when tail object is written and also when
its corresponding head object is deleted (like here in this case).

Also done some cleanup.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>