[SignalR] Allocate one StreamItemMessage per stream

[BrennanConroy]

Allocate a single StreamItemMessage per stream instead of per stream item

Unfortunately this does add a new API so will need to go to API review.

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[davidfowl]

Split the changes so we can API review just the relevant one.

[BrennanConroy]

d3ba6ab

[davidfowl]

I meant so I can merge this PR

[BrennanConroy]

lol, fine, I'll open a different PR.

[davidfowl]

Would a derived internal type work instead of changing the public one? Item would need to be virtual.