Need to be able to make keep alive connections not shared on a per-transaction basis

I want to be able to be able to mark keep alive connections as not shared on a per-transaction (really per-origin server) basis. There is a config option proxy.config.http.share_server_session but making it be per-transaction is not quite enough because the code that checks that in HttpServerSession::release() doesn't have a transaction any more. So I thought that it was easier to just make HttpServerSession.private_session settable from the plugin.