driver: Find's query argument should be more consistent

At present, it allows:

> If query is a string, []byte, or [encoding/json.RawMessage], it should be treated as a raw JSON payload. Any other type should be marshaled to JSON.

This should be made more consistent.  Either by passing a pre-parsed selector of some type (perhaps a *mongo.Selector?) or the raw JSON only.

Relates to #1014, and probably depends on it.