Move kernel ping method to platform packages #490
Description
Per #489, we have some legacy / test-scaffolding features in the kernel that have no place there anymore. One of these is the ping method. We still need it, but only to check the health / liveness of worker processes. That being the case, we will move the kernel implementation of method (and the notion of "kernel commands") from the kernel to the extension and nodejs packages.
The ping method is also implemented by the vat (specifically the supervisor). However, this method is of a different character than the other vat RPC methods (i.e. deliver and its friends), which are subject to the kernel queue / cranks. Therefore, we need to separate the vat ping method from the other RPC methods, although the kernel should still be able to ping specific vats, probably via a ping() on VatHandle.