Single proxied

Previously, each websocket connection to the proxy corresponded to a dedicated unix socket connection to the backend. This causes scalability issues and prevents async features like push notifications from being implemented in a reasonable way.

This patchset switches to a single, long-lived unix socket connection that is independent of the number of websocket connections, adding some in-band signaling to allow the backend to be blind of the exact number of clients.

It additionally implements push notifications atop the new architecture, allowing push notifications to work in jailed instances.