Since we will only have builtin extensions for a while, the
more-complicated, watchdog-based extension duplication prevention has
been replaced with one that treats the extension URL as equivalent to
the extension ID. This is true only for built-in extensions, so once we
start supporting third-party extensions we will likely need to bring
back the watchdog (commit e4381b4693) and
the more-complex duplication prevention (commit
670e51d335).
The code here should be treated as a last resort: ideally the GUI should
check if a particular extension is loaded (maybe by calling the
Extension Manager's new `isExtensionLoaded` method) and avoid calling
`loadExtensionURL` if it is.
If an extension worker does not complete initialization in a reasonable
amount of time (currently 15 seconds) then the worker will be
terminated. A complete initialization includes registering an extension.
Central dispatch and worker dispatch share most of their code now by
inheriting from a new shared dispatch class.
Also, the format of passed messages has been altered to make it easier
to understand in a debugger.
