SERVER-10026 port FindingStartCursor dbtests to OplogStart

The FindingStartCursor has been replaced by the OplogStart stage in the new query framework. OplogStart is responsible for traversing backwards through oplog extents in order to find where to start replaying the oplog from. This change ports dbtests for FindingStartCursor such that they test the new query system. It is largely a dbtest addition, but OplogStart has also been instrumented slightly in order to facilitate testing.