proposal: x/net/http2: deprecate WriteScheduler

[neild]

This issue is part of a project to move x/net/http2 into std: #67810

The http2.WriteScheduler interface type defines a scheduler used to select the order in which data is written to HTTP/2 streams. The Server.NewWriteScheduler configuration setting selects a write scheduler to use for server connections. Client connections do not permit configuring the write scheduler.

The package includes three implementations of this interface: Round robin (the default), random, and priority. The priority write scheduler is known to be buggy and the HTTP/2 stream prioritization scheme it implements is deprecated; see #58804 (comment).

Support for user-defined write schedulers is used seldom, if ever, and substantially constrains the HTTP/2 implementation.

Users should not need to configure the HTTP/2 implementation at this level. Of the current set of scheduler options, there is a clear correct choice: The round robin scheduler outperforms the random scheduler, and the priority scheduler is buggy and we have no plans to fix it. If we come up with a better scheduler, we should just make it the default. If we do discover a reason for users to configure this, we can and should provide a simpler configuration setting than what exists now.

I propose deprecating the write scheduler selection mechanism, consisting of the following types and functions:

• FrameWriteRequest
• NewPriorityWriteScheduler
• NewRandomWriteScheduler
• OpenStreamOptions
• PriorityWriteSchedulerConfig
• Server.NewWriteScheduler
• WriteScheduler

I propose that we add a deprecation notice to these types and functions today, and remove support for them in one year. After removing support, servers will ignore the value of the NewWriteScheduler field.