Resolve #7889 avoid 3 fetch calls to load a notebook

[ikiw]

1. Resolve 3 checkpoint get requests invoked to load a single notebook #7889 to avoid 3 fetch calls to load a notebook.
2. Refactored my old PR Resolve #7874 - stop too many fetch calls to docmanager-extension on … #7879 to use debouncer instead of boolean flags

References

#7889

Code changes

1. listCheckpoints() gets invoked thrice when a notebook is loaded resulting in 3 checkpoint calls. The code change is to have an object- checkpointPromiseQueue that holds the promises with locaPath as key. If there is already a promise to be resolved, the pending promise object is returned back instead of creating one. And the promise reference to be cleared off from the checkpointPromiseQueue, when the promise if resolved/rejected via finally block. And as a result only 1 fetch call to checkpoint is triggered.

2. Refactor of older PR Resolve #7874 - stop too many fetch calls to docmanager-extension on … #7879, to make use of Debouncer in this use case as it solves the purpose of executing the callback only once even though invoked multiple times and making the code more cleaner.

User-facing changes

Backwards-incompatible changes

No

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[afshin]

Hello @ikiw! Thank you for working on this issue! I've been looking at it and thinking about some Poll conversations that came up in the last two weekly calls.

Our weekly calls are schedule at a difficult time for your time zone (Wednesdays at 17:00 UTC at https://calpoly.zoom.us/my/jupyter), so I know you may not be able to come to the call. But your issue ties in with that conversation, so I would like to discuss it on the call today. If you can make it, that would be wonderful. If not, I will post the outcome of our conversation in this issue as well.

cc: @blink1073 @jasongrout @saulshanabrook @wolfv

[ikiw]

Will be creating a new PR for the same from a feature branch. As my master branch is messed up with merge issues.