fix: avoid synchronous calls where possible

[akx]

These were a bunch of *Sync calls that were rather easy to convert to promisified ones, either via util.promisify or just node:fs/promises.

There's at least one more place I didn't touch, solely because it would have required refactoring all of server.js::start to be async.

There are other places left (out of scope of this PR) where e.g. callback async could be converted to promises, etc.

[acalcutt]

I would love another review of this, if anyone has the time. In general I think this is a good direction, but I am not very familiar with promises. I assume this makes things run a bit more in parallel so it would increase response times?

I reached out to @mnutt to see if he can offer a review, but I'm not sure if he is around right now and/or if he needs his permissions to this repo put back like I had to get done.

[akx]

I assume this makes things run a bit more in parallel so it would increase response times?

It should decrease response times (that is, make things faster) where there are multiple concurrent requests to code paths which previously used *Sync calls (since a synchronous call will tie up the entire interpreter).

[acalcutt]

I tested this and it seems to work fine. seems like a good direction to get away from synchronous calls