[PHP-638] Return constant string types in getReadPreference()

Replace numeric types with the constant string values and eliminate the irrelevant "type_string" field.

The change to mongo_read_preference_type_to_name() should also fix compatibility with mongos, as the "preferred" strings were not what the server expected in the cursor $readPreference mode.
[PHP-638] Test $readPreference modes for mongos cursors