rgw/dbstore: Use Object ID to handle racing writes

rgw/dbstore: Use Object ID to handle racing writes

Create unique ID for each object upload which will be
atomically updated in the head object at the end. This will
prevent data corruption during concurrent writes.
Incase of Multipart Uploads, upload_id is used as ObjectID.

XXX: The stale or obsolete tail data needs to be deleted

Also addressed invalid usage of CephContext in dbstore tests.

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