MessageCache: Avoid 'contributions' replicas

On WMF wikis, the partitioning of the revision table on the
'contributions' replicas makes the query here perform really poorly.
Specify 'api' as a hack for now to avoid those replicas.

This query happens once per day per wiki, plus when someone edits the
MediaWiki namespace, so it shouldn't be much additional load.

Bug: T164666
Change-Id: I5ae74d1702144f6475e9cfb13effc43389d66233