castComplexIds() should work for any query

[bengourley]

Currently it only iterates the top level properties of a query object:
https://github.com/serby/save-mongodb/blob/master/lib/mongodb-engine.js#L71-L85

Here is a link to a more complete implementation:
https://github.com/Automattic/monk/blob/7fb51e6949dadfaec8e41da5a1d01127f749a2dc/lib/collection.js#L547-L581

[domharrington]

We should also remove the exposed idType() function and allow it to be customised so that it can be overridden for custom uuid formats.