90% - 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set to primary as it is used when generating views, and reading from secondary may cause a view to be generated with out-of-date data if the replication lag is too high.

Removed ``MongoTripod::getReadPreference`` public method - this was only added in for the purposes of unit testing, but we can use the $db property instead.  This is still not great to have it public but we decided modifying that is outside the scope of this PR.