Add JSON-RPC over Websockets back-end in Project API

[vparfonov]

For now our client (IDE) communicate with Project Service via REST in same time we receive notification from File Watching System via JSON-RPC over Websockets. This situation complicate our model and hard for documentation, so we decide to switch client communication to the JSON-RPC calls.

Under this issue need to:

• add JSON-RPC handlers/transmitters on server-side;
• deprecate ProjectServiceClient in IDE client API;
• replace ProjectServiceClient with JSON-RPC calls
• remove deprecated classes

[gazarenkov]

@dkuleshov could you please draft a short specs (right here) about how we replace REST with JSON-RPC for Project API, it is really interesting and important exercise since there are a lot of different methods.

[dkuleshov]

@gazarenkov here is a spec draft #6219