Filter equality matchers on the server (dynamo) side

[aaron7]

Index optimisation to minimise DynamoDB reads per query. Part of #209.

Expect query latency to decrease a small amount for queries with specific filters such as {foo="bar"}, especially those with many entries which do not match.

Will measure before and after on grafana. Could introduce performance tests to test how much chunks fetched from the store for particular queries and stored chunks.