Transient connector implementation

Based on previous discussion here #264, I decided to implement the functionality as a Connector instead.

This has the benefit that all the internals used by relations, events, hooks and so forth still work without any modifications. In addition you can define your own implementation of the connector's `generateId` method, which by default generates random numbers or mongo-compatible random hex strings. You can also set a Model's settings to `idInjection: false` to not set any Id at all.