Combine public/private API endpoints from services about exonum HOT 1 CLOSED

@defuz rename to public/private api endpoints.
As i see it, each node should to have 2 api ports:

1. for handling public requests from anyone (all public endpoints from all services combined here).
2. for handling private requests from node maintainer/s (all private endpoints from all services combined here). The only case this is actual now is for configuration service: the node signs propose or vote with its machine key on behalf of the entity, who put request onto private endpoint.
   If not, we may have to add other means of authentication and, probably, combine with #62

@slowli

from exonum.