Add an option to shutdown when connections reach zero

*Description*:
For graceful shutdown scenarios described in #1990 we largely have what we need, but it would be extra helpful to allow envoy to exit once ingress connections reach zero.

Basically if we have a service that wants to:
1. Use envoy for ingress and egress
2. Has a potentially long and highly variable drain time (e.g. 5 min to 4 hours)

It would be nice if we could have a sigterm:
1. unbind from ingress ports and begin draining the connections.
2. leave egress open for any inflight requests
Note: #1 and #2 are already supported
3. Once all of the ingress (or all!) connections have closed then have envoy exit.

*Relevant Links*:
https://github.com/envoyproxy/envoy/issues/1990