Refactor validator server naming and modules

[sergerad]

Stacked PR.

Creates clearer separation between validator server scaffolding (ValidatorServer) and api impl / business logic (Validator).

• Validator renamed to ValidatorServer (has ValidatorServer::serve()) and ValidatorServer renamed to Validator
• validator module added to validator server module
• error renamed to ValidatorError

From

server/
    mod.rs
    sign_block.rs
    validate_block.rs
    ...

To

server/
    mod.rs
    validator/
        *.rs