rpcdaemon: extract JSON-RPC handler out of transport

[canepat]

This PR performs some refactoring to remove any dependency from http and ws modules to json_rpc module to avoid transport-level being dependent on presentation-level, which is kind of an anti-pattern. Namely, a new transport module is introduced which defines the rpc::RequestHandler interface and will contain both http and ws modules in the future.

This PR is the first one in a row.

Extras

• improve readability in http::Server run loop
• move StreamWriter from common to transport
• improve naming in ws::Connection
• demote some log traces