Add strictness within the HTTP2 window-update STM.
Add strictness within the HTTP2 window-update STM.

kazu-yamamoto/wai,yesodweb/wai,creichert/wai,kazu-yamamoto/wai,creichert/wai,sordina/wai,yesodweb/wai,sordina/wai,sordina/wai,sordina/wai