Terminating function pods should attempt a graceful shutdown.

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Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE REQUEST

What happened:
When pods are terminated, active connections are closed.

What you expected to happen:
The function pod should attempt a graceful shutdown on sigterm with some configurable timeout.

How to reproduce it (as minimally and precisely as possible):

1. Create a function that sleeps for 10 seconds or so and returns success.
2. Trigger the function.
3. Intentionally terminate the pod/docker container before the function completes

You'll get a socket hang up error.

Anything else we need to know?:
I might like to attempt a PR for this if you decide it's an feature worth having.
From what I can see it looks like it could be added relatively easily to the function proxy following this example: https://gist.github.com/peterhellberg/38117e546c217960747aacf689af3dc2

Environment:

• Kubernetes version (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.0", GitCommit:"70132b0f130acc0bed193d9ba59dd186f0e634cf", GitTreeState:"clean", BuildDate:"2019-12-07T21:20:10Z", GoVersion:"go1.13.4", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"13+", GitVersion:"v1.13.12-gke.16", GitCommit:"d40fd337c7a73c3720b57f23b8d1f21f1b2df7ca", GitTreeState:"clean", BuildDate:"2019-11-25T19:40:16Z", GoVersion:"go1.12.11b4", Compiler:"gc", Platform:"linux/amd64"}

• Kubeless version (use kubeless version): Kubeless version: v1.0.4-dirty
• Cloud provider or physical cluster: GKE

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This issue can be closed.