Optimize consistency and state handling

Description

This optimizes how BuildStream handles states, and reduces the number of calls that are necessary to detect whether something is cached or not.

Changes proposed in this merge request:

• add a is_resolved on sources to determine whether the source is resolved or not. Default implementation is self.get_ref() is not None
• add a is_cached() on sources to determine whether the source has a local copy of its data
• Remove get_consistency() on Source, which only allows coarse-grained state tracking and is not needed by 1 and 2.
• separate 'in source cache' and 'cached by plugin' logic in Element

Fixes

Fix #1126 (closed)

BREAKING BEHAVIORS

This optimization ensures that we do not call the check for a plugin of whether its sources are cached or not.

This means that if sources are in the internal source cache, we don't call the validate_cache part of the plugin anymore, which means that warnings will not be shown/will not error the build as soon as the source is in the source cache.

Since this is a breaking behavior it will need to be synchronized with

Benchmarks

Using the dummy debian project

• Finished in: 84 mins, 32 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-consistency - 8b8b7e52	81.445	81.20 ± 0.74
	py37	master - 51263e21	84.735	84.74 ± 0.22
build - 24	py37	bschubert/optimize-consistency - 8b8b7e52	81.35	81.28 ± 0.35
	py37	master - 51263e21	84.975	85.04 ± 0.47
build - 4	py37	bschubert/optimize-consistency - 8b8b7e52	88.775	88.84 ± 0.26
	py37	master - 51263e21	92.18	92.07 ± 0.76
build - 8	py37	bschubert/optimize-consistency - 8b8b7e52	81.725	81.86 ± 0.48
	py37	master - 51263e21	84.985	84.84 ± 0.48
show	py37	bschubert/optimize-consistency - 8b8b7e52	4.84	4.84 ± 0.03
	py37	master - 51263e21	4.89	4.90 ± 0.04
show - cached	py37	bschubert/optimize-consistency - 8b8b7e52	7.525	7.53 ± 0.32
	py37	master - 51263e21	7.93	8.00 ± 0.16
show - sources fetched	py37	bschubert/optimize-consistency - 8b8b7e52	7.135	7.08 ± 0.23
	py37	master - 51263e21	4.87	4.87 ± 0.05
source fetch	py37	bschubert/optimize-consistency - 8b8b7e52	41.695	41.68 ± 0.27
	py37	master - 51263e21	48.055	47.92 ± 0.33

There were 4 runs of each command.

Using a real internal project

action	commit	real	user	sys
bst show - nothing cached	master	1m28	1m47	0m17
		1m34	1m53	0m18
		1m32	1m50	0m17
	optimize-consistency	0m11	0m10	0m01
		0m11	0m10	0m01
		0m11	0m10	0m03
bst --fetchers 32 source fetch	master	5m01	24m4	11m35
		5m05	23m55	11m34
		5m04	24m6	11m31
	optimize-consistency	4m11	24m33	11m2
		4m05	23m44	10m40
		4m16	24m53	11m11
bst show - sources cached	master	1m24	0m31	0m55
		1m21	0m31	0m53
		1m24	0m32	0m56
	optimize-consistency	0m17	0m13	0m03
		0m16	0m13	0m03
		0m18	0m14	0m03
bst build	NOT COMPUTED, TOO LONG	X	X	X
bst show - all cached	master	1m58	0m38	1m32
		1m54	0m37	1m20
		1m56	0m38	1m21
	optimize-consistency	0m26s	0m16s	0m06
		0m19	0m15	0m04
		0m19	0m15	0m04

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