Memory leak

I was banging my head trying to figure out why my Node 12.8.3 process' memory footprint was increasing indefinitely and managed to narrow down the problem to this module (used by @godaddy/Terminus in my case).

For some reason, the server._pendingSockets map would not be properly maintained, keeping references to thousands of requests.

As soon as I switched from using stoppable to http-terminator, the problem went away.

Please note that I am using node-spdy for the http2 server, since the core implementation is not yet supported by express.