API: Use rev_user when possible for list=usercontribs

API: Use rev_user when possible for list=usercontribs

When all ucusers are registered users, we can do it somewhat more
efficiently by querying by rev_user instead of rev_user_text. This
should particularly make a difference on the WMF cluster where the
revision table is clustered by rev_user on the relevant slaves.

This also kills the forcing of the usertext_timestamp index in the
query. MySQL should be smart enough these days to not need that.

Bug: T131065
Change-Id: Ie8317ea15937f98133d4c1b69315c35aa2e046fa