Don't use guid in GetRequestIdentifier

[halter73]

Guid.NewGuid and Guid.ToString are a little slow. Using a incrementing long instead with an random starting seed will likely be faster.

We should also look at the most efficient way to generate a string from this long.

[Eilon]

We've had a lot of discussion before on Guid vs. other types of ids. Could we still use a Guid but generate it ourselves? Just thinking out loud here.

[Tratcher]

@halter73's description sounds like what System.Web did previously.

You could also make Kestrel generate it directly using something like the socket handle and a request counter.