fix(storage/mongodb): add claimed index hint

This gives a hint to the mongodb query optimizer to use the correct
index when dealing with POST claim requests. It results in 4x fewer
objects being scanned, 7x fewer index entries scanned, and loads only
one index rather than all of them.

Change-Id: Ia48a0953b843663a4d80219c05d4a98f3e3ee6b0