RevisionSpec is missing concurrencyModel

[mattmoor]

From: #444

concurrencyModel: SingleThreaded

This should be added here and (possibly?) used to configure the ela-queue container here.

@josephburnett @mdemirhan How do you see us enforcing the concurrencyModel?

cc @bobcatfish @cooperneil

[mdemirhan]

Joe probably has more context here. He was looking into this from autoscaler's perspective. @josephburnett - any comments?

[mdemirhan]

Also, regarding ela-queue, I will add a work item to see if it can be replaced by Envoy completely and if we can still do concurrency & timeout enforcement.

[bobcatfish]

Whoever works on this please ping me re. adding this to the conformance tests :D

[mattmoor]

/assign @josephburnett

@bobcatfish Joe has a PR with this in progress.

[josephburnett]

Done: #749