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.

rpc/errors.go

@@ -18,18 +18,31 @@ package rpc
 
 import "fmt"
 
-// request is for an unknown service
-type methodNotFoundError struct {
-	service string
-	method  string
-}
+const defaultErrorCode = -32000
+
+type methodNotFoundError struct{ method string }
 
 func (e *methodNotFoundError) ErrorCode() int { return -32601 }
 
 func (e *methodNotFoundError) Error() string {
-	return fmt.Sprintf("The method %s%s%s does not exist/is not available", e.service, serviceMethodSeparator, e.method)
+	return fmt.Sprintf("the method %s does not exist/is not available", e.method)
 }
 
+type subscriptionNotFoundError struct{ namespace, subscription string }
+
+func (e *subscriptionNotFoundError) ErrorCode() int { return -32601 }
+
+func (e *subscriptionNotFoundError) Error() string {
+	return fmt.Sprintf("no %q subscription in %s namespace", e.subscription, e.namespace)
+}
+
+// Invalid JSON was received by the server.
+type parseError struct{ message string }
+
+func (e *parseError) ErrorCode() int { return -32700 }
+
+func (e *parseError) Error() string { return e.message }
+
 // received message isn't a valid request
 type invalidRequestError struct{ message string }
 
@@ -50,17 +63,3 @@ type invalidParamsError struct{ message string }
 func (e *invalidParamsError) ErrorCode() int { return -32602 }
 
 func (e *invalidParamsError) Error() string { return e.message }
-
-// logic error, callback returned an error
-type callbackError struct{ message string }
-
-func (e *callbackError) ErrorCode() int { return -32000 }
-
-func (e *callbackError) Error() string { return e.message }
-
-// issued when a request is received after the server is issued to stop.
-type shutdownError struct{}
-
-func (e *shutdownError) ErrorCode() int { return -32000 }
-
-func (e *shutdownError) Error() string { return "server is shutting down" }