ingester: use consistent set of instances to avoid panic

[bboreham]

Calling getInstances() twice runs the risk that the set has expanded or contracted between allocating the slice and starting to iterate.

Stack trace where it went wrong:

panic: runtime error: index out of range [1743] with length 1743
goroutine 514 [running]:
github.com/grafana/loki/pkg/ingester.newStreamsIterator({0x27cc280, 0xc000a73600})
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:218 +0x2ba
github.com/grafana/loki/pkg/ingester.(*ingesterSeriesIter).Iter(0x17c33cf)
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:189 +0x25
github.com/grafana/loki/pkg/ingester.(*Checkpointer).PerformCheckpoint(0xc127edff78)
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:579 +0x25f
github.com/grafana/loki/pkg/ingester.(*Checkpointer).Run(0xc127edff78)
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:618 +0x21c
github.com/grafana/loki/pkg/ingester.(*walWrapper).run(0xc0009a3810)
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/wal.go:166 +0x21c
created by github.com/grafana/loki/pkg/ingester.(*walWrapper).Start
	/src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/wal.go:102 +0x6f

Checklist

• NA Documentation added
• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

(I didn't add a test as it would be complex to synthesise delays in fetching the set to recreate the race.)

[cyriltovena]

Lgtm