On connect and disconnect

We'll need a JS-based StatsD library (and we might need to upgrade our server StatsD version to actually receive these, though that doesn't matter for this).

I assume we'll want to base this on #2.

So that we don't DoS ourselves with key requests when a server restarts

We're sending pings to the client, but we're not checking for the response. It's possible that this is causing us to not properly detect broken connections, which would presumably cause a memory leak. We should check for a pong as explained in the ws docs.

See WIP code in the https://github.com/zotero/stream-server/tree/uws-to-ws branch

request() doesn't throw on a non-200 status codes, so our handling code in zotero_api.js is wrong.

A deleteSubscriptions request for a public or global topic without an API key results in an 'apiKey' not provided error. This was an existing bug before global topics, but it prevents global topics from being deleted.

Right now a restart or shutdown triggers lots of activity as all connections are deregistered. This can take a while when there are lots of connections, and the delay means that health checks can fail, causing the instance to be taken out of service.

We should do the bare minimum to stop accepting new connections and close all connections cleanly before shutting down.