Investigate Area.metadata.leftRightIndex index

[vnugent]

When reviewing PR #296 I happened to look at the area schema and thought the current index on leftRightIndex is incorrect (we're not just searching for a leftToRightIndex alone). It should be a compound index with both _id and metadata.leftRightIndex.

https://github.com/OpenBeta/openbeta-graphql/blob/develop/src/db/AreaSchema.ts#L120

To do:

• add .explain('executionStats') and see how it affects sorting

[vnugent]

#376