Merge pull request #166 from cybozu-go/refactor/move-helper-to-api-pkg #822

Job	Run time
build (3Mi, incr-backup)	31m 18s
build (3Mi, full-backup)	31m 30s
build (3Mi, change-to-standalone)	25m 29s
build (3Mi, change-to-primary)	27m 34s
build (3Mi, change-to-secondary)	31m 21s
build (200Gi, various-transfer-part-size)	20m 36s
build (3Mi, backup-failure)	27m 57s
build (10Mi, various-transfer-part-size)	20m 31s
build (3Mi, !//)	34m 13s
	4h 10m 29s

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