Only increment status message cursor if we're going to consume the message

Summary:
Fixes the long uptake we saw on meta.phacility.com. I regressed this in D11795.

We make three calls to this method, but only one actually consumes the messages. The other two are just checking to see if there are any messages.

Only move the cursor up if we're actually going to process the messages.

Test Plan: Sort of tricky to test convincingly since it's inherently race-prone, but ran debug pulllocal and pushed update messages and saw it pick them up.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11808