Add support for HttpWebRequest.ReadWriteTimeout

[stephentoub]

Uses the recently added support for SocketsHttpHandler.ConnectCallback to be used for synchronous operations (#45300) to add support for HttpWebRequest.ReadWriteTimeout. That value is now used to set the Read/WriteTimeout on the underlying socket, with it then affecting all synchronous reads and writes on that connection. The ReadWriteTimeout is added to the cache key for whether the cached HttpClient instance can be used, as is whether the request is sync or async because that influences how connections are created from the HttpClient instance.

Fixes #43520

A couple of notes:

• If we had API Proposal: SocketsHttpConnectionContext.ConnectAsynchronously #44876, the Async flag could be removed from the cache key and the same instance could be reused for both sync and async operations, because whether the connection was being initiated from a sync or async request would be passed into the callback.
• By switching from HttpClientHandler to SocketsHttpHandler, HttpWebRequest no longer by default uses a DiagnosticHandler. That's Customized SocketsHttpHandler does not support diagnostics tracking #31261.

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Uses the recently added support for SocketsHttpHandler.ConnectCallback to be used for synchronous operations (#45300) to add support for HttpWebRequest.ReadWriteTimeout. That value is now used to set the Read/WriteTimeout on the underlying socket, with it then affecting all synchronous reads and writes on that connection. The ReadWriteTimeout is added to the cache key for whether the cached HttpClient instance can be used, as is whether the request is sync or async because that influences how connections are created from the HttpClient instance.

Fixes #43520

A couple of notes:

• If we had API Proposal: SocketsHttpConnectionContext.ConnectAsynchronously #44876, the Async flag could be removed from the cache key and the same instance could be reused for both sync and async operations, because whether the connection was being initiated from a sync or async request would be passed into the callback.
• By switching from HttpClientHandler to SocketsHttpHandler, HttpWebRequest no longer by default uses a DiagnosticHandler. That's Customized SocketsHttpHandler does not support diagnostics tracking #31261.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[wfurt]

LGTM