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.
This also changes timeout error format to
"page.click: Timeout 5000ms exceeded", so that all errors
can be similarly prefixed with api name.
We can now have different api names in different clients,
and our protocol is more reasonable.
