Fixes T6713. The idea is to keep checking what's going on in the update paths that touch the DOM. If we're doing an update or should be doing a different update, then we bail early.

This is the type of code + testing that makes me dizzy after awhile, but I think it works...

added a "forceStall" parameter to the column view controller, which when specified sleeps for seconds before returning. I then augmented the JS such that the "send message" code for the durable column would specifiy this parameter.

For actual testing, I then spammed the heck out of the durable column channel and saw each message only once. I also spammed the column, switched browsers to a user on the same thread in the normal "speedy" view, sent messages there, and also only received one copy