Fixes for asyncio loops were we don't follow the documentation

Description

This modifies the scheduler and job implementation to spawn new processes and handle callbacks according to the documentation

Benchmarks

We can see that the performances are not worse, even slightly better in some cases.

• Finished in: 102 mins, 46 secs
• Number of builders: [4, 8, 12, 24]
• Target files: base-files/base-files.bst
• Number of runs: 4
• Number of warmups: 2
• Python versions: py37

action	python_version	commit	median time (secs)	mean time (secs) ± std
build - 12	py37	bschubert/stricter-asyncio-handling - af8f1c3d	119.805	119.66 ± 0.55
	py37	master - 454ed339	121.79	121.81 ± 0.28
build - 24	py37	bschubert/stricter-asyncio-handling - af8f1c3d	118.555	118.48 ± 0.19
	py37	master - 454ed339	120.215	120.25 ± 0.39
build - 4	py37	bschubert/stricter-asyncio-handling - af8f1c3d	120.91	121.01 ± 0.62
	py37	master - 454ed339	122.175	122.11 ± 0.42
build - 8	py37	bschubert/stricter-asyncio-handling - af8f1c3d	122.375	122.23 ± 0.84
	py37	master - 454ed339	125.105	125.01 ± 0.31
show	py37	bschubert/stricter-asyncio-handling - af8f1c3d	8.03	8.01 ± 0.15
	py37	master - 454ed339	4.875	4.87 ± 0.03
show - cached	py37	bschubert/stricter-asyncio-handling - af8f1c3d	8.195	8.14 ± 0.20
	py37	master - 454ed339	8.18	8.14 ± 0.18

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