Use identity_token for refresh(), unexpire, undefer

Use identity_token for refresh(), unexpire, undefer

The horizontal sharding extension now makes use of the identity token
added to ORM identity keys as part of :ticket:`4137`, when an object
refresh or column-based deferred load or unexpiration operation occurs.
Since we know the "shard" that the object originated from, we make
use of this value when refreshing, thereby avoiding queries against
other shards that don't match this object's identity in any case.

Change-Id: Ib91637a65d94ace7405998b8410d62944a83f2eb
Fixes: #4247