Persistent listeners for onMessage and onConnect are special because they
don't have parent EventManagers, so we have a lot of custom code just to keep
track of which extensions have registered them during previous runs.

In this patch, I simplify all that logic to always assume that any extension
which sends messages from content scripts has those listeners setup properly.

The only observable difference is that some poorly written extensions which
were previously broken by delayed startup will now "work" by being started
earlier if a message from a content script arrives during browser startup.

Additionally, unconfuse two different meanings of the "startup" event.
Bonus, avoid some logspam during tests.

Differential Revision: https://phabricator.services.mozilla.com/D70955