Add :start! and :continue! for strict process routing replies to process manager interested?/1 callback.

Strict routing enforces the following validation checks:

• {:start!, process_uuid} - validate process does not already exist.
• {:continue!, process_uuid} - validate process already exists.

If the check fails an error will be passed to the error/3 callback function:

• {:error, {:start!, :process_already_started}}
• {:error, {:continue!, :process_not_started}}

The error/3 function can choose to :stop the process or :skip the problematic event.

Fixes #212.