rpc: implement full bi-directional communication (#18471)

New APIs added:

    client.RegisterName(namespace, service) // makes service available to server
    client.Notify(ctx, method, args...)     // sends a notification
    ClientFromContext(ctx)                  // to get a client in handler method

This is essentially a rewrite of the server-side code. JSON-RPC
processing code is now the same on both server and client side. Many
minor issues were fixed in the process and there is a new test suite for
JSON-RPC spec compliance (and non-compliance in some cases).

List of behavior changes:

- Method handlers are now called with a per-request context instead of a
  per-connection context. The context is canceled right after the method
  returns.
- Subscription error channels are always closed when the connection
  ends. There is no need to also wait on the Notifier's Closed channel
  to detect whether the subscription has ended.
- Client now omits "params" instead of sending "params": null when there
  are no arguments to a call. The previous behavior was not compliant
  with the spec. The server still accepts "params": null.
- Floating point numbers are allowed as "id". The spec doesn't allow
  them, but we handle request "id" as json.RawMessage and guarantee that
  the same number will be sent back.
- Logging is improved significantly. There is now a message at DEBUG
  level for each RPC call served.