fix(storage.mongodb): Race condition when creating a claim

This patch forces MessageController.claimed to always query the primary in
order to resolve a race condition in ClaimController.create. The latter
method tags a batch of messages with a claim ID, then immediately
calles claimed() to get a list of messages that were actually updated.

MessageController.claimed is also used by ClaimController.update, so that
request will no longer read from secondaries either. This shouldn't put
much more load on the primary, and may even be desired behavior in the case
that a client creates a claim and immediately attempts to update it, in
which case they could get a "not found" if the new claim hasn't propagated
yet to the secondary being queried.

Change-Id: I8b5cfee1cc6f185ebf53e98fad95a5fc30e021da
Partial-Bug: #1218990
Merge "fix(storage.mongodb): Race condition when creating a claim"