Add in memory tag and metastore index

*STILL A WORK IN PROGRESS*

## Overview

This PR adds an in memory metastore and tag index. It should be treated like a write through cache for the metastore. Whenever the metastore gets updated the index should. The index is loaded completely into memory on server load.

It will provide quick lookups for measurements, series, and filtering those based on tags and regexes.
OK, I think it's be worth trying to come up with name. We all know naming is hard, but a different name might be quite helpful for other devs.