Investigate RegionKey for rsID lookups

[abought]

h/t @weinstockj

The current rsID lookup feature is based on a somewhat intricate internal structure of position: {refalt: rsid}. We might be able to simplify internals (and serialization) with a clever approach called VariantKey.

Items to evaluate:

• Performance
• Storage space used
• Conceptual readability of code
• Preservation of internal sort order (for efficient use of LMDB cache)

https://github.com/Genomicsplc/variantkey