kernel: Upgrade VatWorkerService from interface to abstract base class.

[grypez]

The existing VatWorkerService is a platform agnostic interface, which the extension implements using iframes and message ports. There are a few details (promise management, delete all calling forEach(delete)) which can be implemented agnostically. Once we are ready to target another platform, e.g. node, it would be nice to pull these details (and their tests) out into the @ocap/kernel package by upgrading VatWorkerService to an abstract base class.

Previous Work

#131

[grypez]

The existing implementations of the service are dissimilar. For example, the nodejs implementation is entirely captured by a single class, while the extension implementation is split between a client and server. From the kernel's perspective, the client is the service, and the server is expected simply to 'be up'.

Management of vat worker state occurs in the server. This is the thing to be abstracted by a base class.

The interface the kernel demands is specified on the client. This is what the 'extends' or 'implements' keyword would buy us.

We could define weak Client and Server abstractions separately, but then the NodejsVatWorkerService would need to implement the Client and extend the Server.

There doesn't seem to be a strong case for abstraction beyond the VatWorkerService type the kernel already uses.

There is a small case for changing the interface to accept an options bag in the constructor, since the implementations are so different.