_scheduler/jobs: refactor, defensive send_message

Simplify the custom 'handle_message' / 'send_message' protocol by not requiring a message_type. These message types share a namespace with the base Job implementation, which could cause trouble.

Introduce a new private '_send_message' to implement the old functionality.

Subclasses are free to pack a message type into their own messages, this isn't necessary at present and simplifies existing subclass code.

Also fix some minor lint issues in this file.

Depends on !1334 (merged)