Configurable upstream idle_timeout

[esmet]

Please describe your use case / problem.
I'd like to configure how long keepalive connections are kept for upstream clusters.

Describe the solution you'd like
We can set the upstream cluster timeout on https://www.envoyproxy.io/docs/envoy/v1.9.0/api-v2/api/v2/core/protocol.proto#envoy-api-msg-core-httpprotocoloptions in v2/v2cluster.py

Describe alternatives you've considered
There is currently no alternative. I believe there is no upstream timeout at all unless one is set on the cluster object.

Additional context
I think it would make sense to allow this to be per mapping, but since it would mean the generation of a new cluster for every distinct timeout, this might not be possible/advisable. Is that right? Either way, the ability to set this globally would be enough satisfy my use case in the near term.

[esmet]

I currently have a naive implementation but would be willing to clean it up before submitting a PR

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[kflynn]

This actually got fixed -- thanks @esmet!