Reword doc blocks for Cursor hydrate property/method
Support read preferences in Query with internal UoW hints property

Create UoW hints property, and use that to store refresh, slaveOkay, and read preference hints. Replace cursor-wrapping in Query::execute() with logic in prepareCursor().
Relax assertions in SlaveOkayTest

The previous assertions were too fragile, as they would fail if another hint was present in the array. Changed the assertions to only consider the slaveOkay hint.