This adds one more protocol message __dispose__
to dispose a scope and all child objects.
Now, client side does not need a notion of scope anymore -
it just disposes the whole object subtree upon __dispose__.
Server, on the other hand, marks some objects as scopes
and disposes them manually, also asserting that all parents
are proper scopes.
Drive-by: fix electron issues, exposed by the test using
waitForNavigation.
Drive-by: mark some tests skip(CHANNEL) that were mistakenly
marked skip(USES_HOOKS).
