Introduce async budget in transaction manager

[emhane]

closes #6290

[mattsse]

I forgot that we don't have a main loop where a single budget would be appropriate

perhaps we should break instead
and also take another look at the function as a whole

[DaniPopes]

nits

[mattsse]

few more nits

[mattsse]

this is now an entirely new logic

I prefer the previous one and then revisit this separately, because this was already reviewed.

so we can make progress with this here

[emhane]

made some changes. now every stream can be prioritised differently. though I advice against it, if you still want, I can disable the budget for tx propagation @mattsse

[mattsse]

can we please roll this back to 35e375e first with the nits applied and then revisit separately?

[emhane]

do we want to update this metric before and after poll the tx fetcher for events? this is how it is on main

[emhane]

@mattsse ? last remaining comment

[mattsse]

we should be able to improve this further, something like your additional design but I'd like to test that a bit more first so we get a better understanding what the actual bottleneck here is, most likely incoming network events
we can track this in an issue