TelemetryEnvironment.jsm's EnvironmentAddonBuilder has a shutdown
blocker that depends on the addons database to have been loaded.
There are two calls to asyncLoadDB() in XPIProvider.jsm that are
supposed to activate the load. Neither of them work:

- XPIProvider calls asyncLoadDB() during quitApplicationGranted.
  But "quit-application-granted" is not always triggered, as seen in:
  https://bugzilla.mozilla.org/show_bug.cgi?id=1601678#c12

- XPIProvider.shutdown() calls asyncLoadDB().
  But shutdown() is only called when TelemetryEnvironment's blocker has
  been released. So this is never reached. More details in:
  https://bugzilla.mozilla.org/show_bug.cgi?id=1601678#c7

To fix the deadlock, asyncLoadDB() is called from profile-before-change,
which is the same phase as the blocker of EnvironmentAddonBuilder.

The two existing calls to asyncLoadDB() mentioned above are obsolete and
have been removed.

---

After the removal of asyncLoadDB() from XPIProvider.shutdown(), the
test_ext_persistent_events.js test started to fail. This is because the
test sends the "sessionstore-windows-restored" notification, for which
XPIProvider has a handler that calls asyncLoadDB(), without awaiting
the result.
Since XPIProvider.shutdown() doesn't await the DB load any more, it is
possible for the DB to be unloaded while being used. This only happens
in tests, because the construction with the TelemetryEnvironment ensures
that the addons database has fully loaded before shutdown() is called.

To resolve this test-only issue, AddonTestUtils.promiseShutdownManager()
has been updated to explicitly wait for the pending _dbPromise if any.

Differential Revision: https://phabricator.services.mozilla.com/D91388