Gracefully shutdown children on termination

Description

This ensures we show graceful termination of workers when we abort a build and cleans up some reporting about missing children.

Fix #1185 (closed) (needs backporting to bst 1.4)

Benchmarks

• Finished in: 206 mins, 23 secs
• Number of builders: [4, 8, 12]
• Target files: base-files/base-files.bst
• Number of runs: 5
• Number of warmups: 2
• Python versions: py37

action	python_version	commit	median time (secs)	mean time (secs) ± std
build - 12	py37	bschubert/graceful-children-sigterm - f59d4e3f	303.34	301.39 ± 4.19
	py37	master - b20ea08f	299.64	300.53 ± 5.26
build - 4	py37	bschubert/graceful-children-sigterm - f59d4e3f	281.67	283.00 ± 3.51
	py37	master - b20ea08f	285.08	283.27 ± 11.26
build - 8	py37	bschubert/graceful-children-sigterm - f59d4e3f	285.07	282.89 ± 6.46
	py37	master - b20ea08f	278.81	277.64 ± 3.13
show	py37	bschubert/graceful-children-sigterm - f59d4e3f	5.65	5.65 ± 0.02
	py37	master - b20ea08f	4.38	4.37 ± 0.01
show - cached	py37	bschubert/graceful-children-sigterm - f59d4e3f	5.69	5.67 ± 0.05
	py37	master - b20ea08f	5.62	5.63 ± 0.02

There were 5 runs of each command.

Overall, it seems like only inconsistencies in the benchmarks rather than real slow down.