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set to primary as it is used when generating views, and reading from secondary may cause a view to be generated with out-of-date data if the replication lag is too high.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.
But that's not a good reason.  Look at my branch.  You can get the preference from db and collection off of tripod.