Improving ETags

[chriswk]

Describe the feature request

I want Edge to perform ETag calculations for client keys and the /api/client/features endpoint when we update features instead of calculating it after calculating the response each time.

Background

Currently we use a lot of memory and CPU on each request, even if we end up yielding a 304, since our current ETag middleware is performing the service call and then calculating the hash based on the response byte stream and then passing the response byte stream onto the user.

Solution suggestions

Since we already have code in place that stores all client keys we know about and a method to evaluate whether a key is subsumed by another, calculating ETags could be done on a per client key basis, with only keys subsumed by the key that received an update needing to be updated.

[chriswk]

Solved by #390 once it gets merged