30% - TENT-609 Make tripod saves set db read preference to primary

We are currently setting the collection read preference to primary when calling MongoTripod::SaveChanges().  However the db object also needs to have it's read preference set to primary as it is used when generating views, and reading from secondary may cause a view to be generated with out-of-date data if the replication lag is too high.

Tests to follow when approach is agreed.
Yes.  I mean, I agree we need to change this, I also agree we don't need to change it in this PR.