EDS requests sent after receiving SIGTERM

**If you are reporting *any* crash or *any* potential security issue, *do not*
open an issue in this repo. Please report the issue via emailing
envoy-security@googlegroups.com where the issue will be triaged appropriately.**

*Title*: EDS requests sent after receiving SIGTERM

*Description*:
When envoy is sent SIGTERM, it will start tearing down clusters and then send new EDS requests dropping those resources.

I am not sure the impact of this, but it seems a bit suspicious at the very least

*Repro steps*:
Send SIGTERM to envoy, turn on trace:config logs and see the discovery requests. Note in my case it was pretty rare to actually see these show up in the XDS server, likely due to racing to shutdown the process, but it does happen. Seems like usually only a couple requests get through for some reason.

*Logs*:
[logs.txt](https://github.com/envoyproxy/envoy/files/5488930/logs.txt)

SIGTERM sent at 07:59:25, you can see the EDS requests at that point