Remove support for multi adapters

This PR is to revert the half-baked multi-adapter support.

The reason behind this decision is due to the fact that there are many problems that we have yet to resolve, which is how can we get association works for 2 models which use different adapter (thinking of 1 using NoSQL and another is SQL). In the end, it'd be the best to put a constraint on this feature by enforcing 1 adapter for all like Rails has been doing. We will revisit this in near future.

TODO:

* [x] Remove `AdapterRegistry`
* [x] Only allow 1 adapter to be registered and loaded
* [x] Document the new DSL

`UserEntity` => `UserRepository` ;)