Convert internal stream types to JSON-RPC

[rekmarks]

We should use JSON-RPC for all kernel IPC. Our discrete units of IPC are the streams and messages sent between two conceptual (and usually actual) parts of the kernel. They are:

• UI panel <-> kernel (aka "kernel control" messages, refactor: Begin using JSON-RPC for internal messages #451)
• Vat worker client <-> vat worker server
• Web extension kernel <-> iframe worker
• Web extension background <-> offscreen document
• Node.js kernel <-> Node.js worker

[rekmarks]

Consolidated into #369.