Probably improve behavior around duplicate notifications

Summary:
We're sometimes getting duplicate notifications right now. I think this is because multiple windows are racing and becoming leaders.

Clean this up a little:

• Fix the timeoout typo.
• Only try to usurp once.
• Use different usurp and expire delays, so we don't fire them at the exact same time.

Not sure if this'll work or not but it should theoretically be a little cleaner.

Test Plan:

• Quit Safari, reopened Safari, still saw a fast reconnect to the notification server (this is the goal of usurping).
• Did normal notification stuff like opening a chat in two windows, got notifications.
• Hard to reproduce the race for sure, but this at least fixes the outright timeoout bug.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15806