[feature-request] Replace std::unordered_map with flat map for http headers

[itrofimow]

Looking at flamegraphs from #141 (comment) one can see that unordered_map takes ~10-11% off total time, with most of it coming from http internals (and some from TimeStorage).

There is an idea that it could very well be replaced with flat map, that would fall back to unordered_map after some threshold, and chances are it would perform better - not only std::unordered_map is pretty slow and linear search might perform better for small number of keys, but there also is that HashDOS-preventing hash, which ain't the fastest. Also my experience tells me that it's not that common to have, say, 50+ headers in http.

What is also promising is that this abstract HttpHeadersMap could be implemented via on stack buffer containing data in http-format right away! - just add some array of string_views which indexes into that buffer. Not sure whether that would work for requests, but should be possible for responses, and for responses we could just pass it's content in Socket::SendAll as is

[apolukhin]

Another idea is to move special headers (like Content-Type, Date, Content-Length) into a separate fields inside a structure to totally avoid search for them

[itrofimow]

Custom map for http headers is merged in c4b1d90