Consider updating HttpClientFactory defaults to leverage SocketsHttpHandler

[rynowak]

For .NET 5 we should switch the defaults of the factory:

• configure a connection lifetime for SocketsHttpHandler
• disable handler rotation

The problem that's solved by the current behavior (handler rotation) is solved more elegantly by just using the feature for it.

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[CarnaViire]

Triage:
We cannot disable/remove handler rotation entirely because of 2 reasons:

• it will have to stay for .NET Framework
• it will have to stay for custom primary handlers as it might not be backed by SocketsHttpHandler at all, e.g. WinHttpHandler

We may still use SocketsHttpHandler for default primary handlers on .NET Core, but the benefit will not be as big (we cannot remove the rotation entirely).

Moving to future.