Adding a new listener to EventPipeEventDispatcher temporarily stops emitting events for existing sessions

[davmason]

In EventPipeEventDispatcher we currently have a shared session for all EventListeners, whenever a new listener is added we will stop the existing session and then start a new session with the combined keywords/level. This has the side effect that for existing listeners we will drop events that happen between stopping the old session and starting the new one, as well as any unread events from the old session when we stop delivering events.

We can address this by changing the logic to update the keywords/level rather than starting and stopping. I have a first attempt at this at https://github.com/davmason/runtime/tree/eventpipe_update