scheduler.py: Optimize scheduling by not calling it unnecessarily

Description

This modifies how we schedule new jobs to optimize the scheduler's loop.

This should reduce the number of missed events we get and reduce the pressure on the _sched method, by calling it less often.

This resolves point 3 of #336 (comment 255606307)

Benchmarks

• Finished in: 92 mins, 33 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/optimize-scheduling - 68323e10	90.81	90.77 ± 0.50
	py37	master - 454ed339	121.425	121.35 ± 0.22
build - 24	py37	bschubert/optimize-scheduling - 68323e10	90.87	90.73 ± 0.42
	py37	master - 454ed339	119.8	119.85 ± 0.16
build - 4	py37	bschubert/optimize-scheduling - 68323e10	98.19	98.20 ± 0.46
	py37	master - 454ed339	121.305	121.32 ± 0.32
build - 8	py37	bschubert/optimize-scheduling - 68323e10	90.65	90.66 ± 0.55
	py37	master - 454ed339	123.96	123.95 ± 0.23
show	py37	bschubert/optimize-scheduling - 68323e10	8.225	7.98 ± 0.59
	py37	master - 454ed339	4.84	4.83 ± 0.02
show - cached	py37	bschubert/optimize-scheduling - 68323e10	8.23	8.19 ± 0.12
	py37	master - 454ed339	8.005	7.90 ± 0.42