Fast-path header dictionaries

[benaadams]

Reopened allocs as #1009

Don't allocate a key string (which then isn't used) for known keys. (Cuts out 42% of string allocations)

Before

After

String allocs are now only x4 higher than other things

Currently the header dictionary is randomly? ordered. This moves things around so some items are in a particular order:

• Host first in FrameRequestHeaders' Append/AppendValueFast/SetValueFast
• Content-Length, Content-Type first in FrameResponseHeaders' AppendValueFast/SetValueFast

Enumerator and CopyToFast in FrameResponseHeaders bail if nothing after: Connection, Date, Server, Content-Length, Transfer-Encoding (for MVC), Content-Type headers - rather than running through the full list of headers.

Full order probably needs work on the rest with frequency analysis.

@nathana1 @halter73 @CesarBS how does this compare to #829 ?

ReponseCookiesFeature and FormFeature moved to delayed allocation in aspnet/HttpAbstractions#675

[halter73]

The first commit seems like an obvious win. Nice catch.

The rest is a little complicated/questionable without benchmarks for the reordering changes specifically showing a nice improvement. I think we're just going to merge the first commit and close the rest.

[benaadams]

@halter73 I think you might be right; looking a dubious difference for request; will close to eval response.

Opened allocs as #1009