Happy Eyeballs support in Socket.ConnectAsync

[scalablecory]

This proposal adds APIs needed to implement RFC 8305 "Happy Eyeballs" in Socket.ConnectAsync. This depends on #939.

The simplified description of Happy Eyeballs is that it makes A and AAAA DNS requests in parallel, and then goes back and forth between A and AAAA beginning parallel Connect() at a set interval until it has a connection. This trades more overhead for improved latency of the root ConnectAsync call.

See https://github.com/dotnet/runtime/issues/26177#issuecomment-540141586 where we decided to explore this as a general Socket feature rather than only SocketsHttpClient.

(API has cancellation support added where there previously hasn't been, to avoid needing to add yet more APIs later. See related #921)

class System.Net.Sockets.Socket
{
    public static bool ConnectAsync (SocketType socketType, ProtocolType protocolType, SocketAsyncEventArgs e, ConnectAlgorithm connectAlgorithm);
}

enum System.Net.Sockets.ConnectAlgorithm
{
    // use existing behavior.
    Default,

    // use a Happy Eyeballs-like algorithm to connect.
    Parallel = 1
}

26177 would be implemented using this, but is not blocked by it.

[scalablecory]

Triage: split dns changes into separate issue, track optimizing existing Connect calls for AF-specific lookups.

[karelz]

Triage: Asked for too many times, we should try to address it in 5.0.

[GrabYourPitchforks]

Feedback:

• Take the new parameter and move it to the end of the parameter list of the new overload.
• Remove [Flags] from the enum. We can add it later if necessary as long as we only have 1 legal value on the enum right now.
• Rename the zero value to Default.
• Consider renaming the enum to ConnectionAlgorithm.

[scalablecory]

Closing for mega-issue #33418.

[paulomorgado]

@GrabYourPitchforks,

From https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/enum:

✔️ DO name flag enums with plural nouns or noun phrases and simple enums with singular nouns or noun phrases.

So, adding or removing the [Flags] attribute has implications on the enum name.