Reduce minetest.after time complexity and provide ordering guarantee

[appgurueu]

Adopted (essentially just trivially rebased) #12043.

The following discussion from @savilli was still unresolved. savilli pointed out that jobs are not completely removed from the data structure, but rather updated to be "dummy" jobs, thus deferring the cost of removing them. The downsides of not removing dummy jobs are the following:

• They make the data structure (slightly) slower for new jobs. This is largely negligible since large n in O(log n) is not an issue.
• They waste space until they expire. This might be an issue if a game heavily relies on jobs being dropped immediately as they are cancelled. Adding a billion jobs expiring in a minute and then cancelling all of them will probably OOM.
• They waste (very little) time when they expire.

For regular use of jobs and cancellation, none of this will be an issue. On the other hand, implementing immediate removal comes with the following issues:

• Maintaining the data structure gets slightly worse constant factors both time- and space-wise. The asymptotic time and space complexity however remain unaffected.
• Much more importantly, the implementation remains rather simple. Immediate removal of jobs would require upgrading the heap to a priority queue and the singly linked lists to doubly linked lists.

[sfan5]

Well if it works feel free to merge it.

[appgurueu]

Had to manually squash and force-push to make sure TurkeyMcMac is properly preserved as the first author in the commit logs.