Consider adding support for .NET Core 2.1 to Kestrel for Memory and Span overloads on Stream

[davidfowl]

It'll remove some of the overhead of getting an array out of the Memory and calling into various Stream APIs.

[muratg]

@pakrym this can be closed?

[davidfowl]

Why?

[benaadams]

int Read(Span<byte> destination);
ValueTask<int> ReadAsync(Memory<byte> destination, CancellationToken cancellationToken = default);
void Write(ReadOnlySpan<byte> source);
Task WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default);