Allow users to change mutistream targets while a stream is active

From a friction log:

> Multistream target currently cannot be updated unless the stream is idle. This presented a problem today during the Playground x Livepeer event.
>
> We were trying to use Livepeer Studio to multistream to Youtube and Beem. However, Beem was having some ingest issues, so the ingest was failing. What I would expect is to be able to restart the Beem multistream target while keeping the Youtube target running. However, I have to keep stopping the entire stream and restarting it. I eventually gave up and only streamed on Beem, and it worked after a few tries.

As a developer, I want to update multistream targets after a stream starts, so that I don't interrupt my viewers experience if I've miskeyed my multistream ingest information or need to restart.

No files, tests, or entry points are named. Trace the multistream target update flow and determine how an active stream can update or restart one target without stopping the others; done means target changes work while the stream remains active and other targets continue running.

Written by the indexing model from the issue text.