Validation of API requests and responses

All API requests and responses must be validated. Validation of requests, responses structure and data types should be extracted to separate validation layer.

Protocol definition should be described in terms of jsonschema. Protocol must be as strict as possible. All required fields must be declared as required, e.t.c.

If request or response is invalid 400 http error is returned with error description and error code in the HTTP response body.

Protocol definition should be bound with the API version.

Steps to implementation:
1. extract validation logic into validation layer,
2. add validation into Node and NodeCollection handlers,
3. make required changes for the validation prototype approvement,
4. add validation into all handlers