invoke, gossip, subscribe, and unsubscribe return whether the operation
was accepted while the shared upstream connection was online. A true value is
not a server acknowledgement.
Pending requests are rejected with a NetworkError when the upstream
connection changes and are not replayed automatically. The server may already
have processed a request whose response was lost, so mutating operations
should be idempotent.
The client emits online when the shared WebSocket opens and offline when
that connection is lost.