Cleanup internal event names

Civi has used Symfony EventDispatcher for internal event-routing since ~4.5. With CRM-19813 and https://github.com/civicrm/org.civicrm.flexmailer making this more visible, inconsistencies in it will become more apparent.

For example, if you look at the list of events emitted through the EventDispatcher, you'll find several different formats for the names:

{{
actionSchedule.getMappings
actionSchedule.prepareMailingQuery
api.authorize
api.exception
api.prepare
api.resolve
api.respond
civi.core.install
civi.flexmailer.compose
civi.flexmailer.run
civi.flexmailer.send
civi.flexmailer.walk
civi.token.eval
civi.token.list
civi.token.render
DAO::post-delete
DAO::post-insert
DAO::post-update
hook_civicrm_caseChange
hook_civicrm_post::Activity
hook_civicrm_post::Case
hook_civicrm_unhandled_exception
}}

I've grepped the universe of published extensions and can't find any which rely on these events, so it seems we still have an opportunity to cleanup the names. I'd like to reduce the number of conventions down to two:

1. For public hooks, use the hook_civicrm_fooBar. This matches the naming used by extensions and CMS modules, and it matches some of the current internal events.

2. For primarily internal events, use dotted convention with camelCase. This seems consistent with most of the current event-names. The main question is whether to use a prefix (like "civi.token.") or not (like "actionSchedule."). Both are common among current events. IMHO, it's a good hedge our bets and use the prefix (in case some future integration winds up blending our EventDispatcher with someone else's EventDispatcher).