Update libuv references in Kestrel docs

[scottaddie]

aspnet/Announcements#296

Per the announcement, managed sockets are replacing libuv in Kestrel as of ASP.NET Core 2.1. Update the docs to mention this and that libuv is used in versions prior to that.

Affected docs:

• https://docs.microsoft.com/aspnet/core/fundamentals/servers/
• https://docs.microsoft.com/aspnet/core/fundamentals/servers/kestrel

[scottaddie]

@guardrex This seems like something you'd enjoy. You want this one?

[guardrex]

Transport Extensibility

The current implementation of the underlying libuv connection semantics has been decoupled from the rest of Kestrel and abstracted away into a new Transport abstraction. While we continue to ship with libuv as the default transport, we are also adding support for a new transport based on the socket types included in .NET.

Socket Transport

We are continuing to invest in a new socket transport for Kestrel as we believe it has the potential to be more performant than the existing libuv transport. While we aren’t quite there yet, you can still easily switch to the new socket transport and try it out today.

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
    .UseSockets()
.UseStartup<Startup>()

[guardrex]

References:

• Remove package dependency from Kestrel to Kestrel.Transport.Libuv aspnet/KestrelHttpServer#2361
• Remove Kestrel.Transport.Libuv from app metapackage aspnet/Universe#959
• https://github.com/aspnet/KestrelHttpServer/pull/2392/files
• Switch default transport to sockets aspnet/KestrelHttpServer#2360