Partition locus/variant/annotation tables by contig

Working in branch **partition_by_contig**

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Then add contig to every query, this would presumably speed up eg gene and variant queries (as we'd have much smaller table to scan)

Remember to look into the new Postgres partitions, if AWS supports it.

Variant - partition by alt = "=" (reference) may be useful

Otherwise you have to partition off a field in that table (ie no relations) so you can't eg partition variant by contig, unless you add contig to the table (duplicating 1/3 of Locus table)

Do at the same time as #12