Adding create_sequence and drop_sequence migration helpers. Fixes #91… #1065

Job	Run time
check_format	1m 56s
specs (shard.yml, 12, 1.6.2, false)	4m 15s
specs (shard.yml, 12, latest, false)	3m 42s
specs (shard.yml, 13, 1.6.2, false)	4m 10s
specs (shard.yml, 13, latest, false)	3m 46s
specs (shard.yml, 14, 1.6.2, false)	4m 10s
specs (shard.yml, 14, latest, false)	3m 46s
specs (shard.edge.yml, latest, 14, true)	3m 52s
specs (shard.yml, nightly, 14, true)	3m 44s
	33m 21s

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