Improve HTTP/2 allocations per request

[davidfowl]

Here's a snapshot of the per request allocations for HTTP/2 requests. I was using https://github.com/codesenberg/bombardier to generate load using 125 connections (1 stream per connection).

Here's 100,000 request for HTTP/1 and HTTP/2 with the same application:

HTTP/1: 24MB

HTTP/2: 650MB

[davidfowl]

I'll do this in preview4. It might be fixed by just pooling #6192

[muratg]

Triage: closing this.