[Epic] HTTP/2 performance

[JamesNK]

In 5.0 we want to make ASP.NET Core 's HTTP/2 performance something we can be proud of. HTTP/2 performance can be advertised as a reason to choose .NET

• Profile HTTP/2 and identify hotspots #13745 Initial HTTP/2 perf investigation
• HTTP/2 benchmarks #13752 HTTP/2 Benchmarks

Identified issues (will grow):

• Pool HTTP/2 Streams on the Http2Connection #6192 Pool HTTP/2 Streams on the Http2Connection
• Add HTTP/2 pseudo-headers as known headers #4761 Add HTTP/2 pseudo-headers as known headers
• HTTP2: Send trailers with data #11270 HTTP2: Send trailers with data
• HTTP2: Reuse request and response Pipe #19276 HTTP2: Reuse request and response Pipe
• HTTP2: Header enumerator allocations #19275 HTTP2: Header enumerator allocations

All open HTTP2 and Perf issues

@Tratcher @halter73 @davidfowl @shirhatti @JunTaoLuo