feat: meteor 2.16

[Julusian]

Update meteor to 2.16, and do some preparation groundwork for 3.0

• move from observe to observeAsync and observeChanges to observeChangesAsync
• replace a few more waitForPromise calls

Notes

This also fixes some concerns I have with some of the *Observer classes. I have concerns about leaks if they are in the middle of executing while stop() is called, to mitigate this I have given each of them a #disposed property which allows various callbacks to know they have been disposed and make sure they don't semi-restart themselves after stop() is called.
The most visible scenario of this is StudioObserver, where if stop was called while its RundownsObserver was being reconstructed, then that RundownsObserver would be leaked.

There was also an issue with some of the debounces, where there could be concurrent executions of them in different fibers if the execution took longer than the debounce time. A new helper has been written and used to mitigate this PromiseDebounce.

I suspect there were also other places of potential leaks, when multiple mongo observers were setup and if one of them threw and error the ones which succeeded would be leaked. The waitForAllObserversReady handler has been added to handle this safely.

TODO:

• Write unit tests for PromiseDebounce, or consider if there is an existing alternative that would work