[REFACTOR] Migrate storage to go-memdb

[toddbaert]

In order to support flag arrays, selection/querying of flags based on flagSetId and other properties, we can migrate to GitHub - hashicorp/go-memdb: Golang in-memory database built on immutable radix trees

This supports in-memory storage and index-based queries, which will allow us to more gracefully handle flags with conflicting keys across sets, and querying/evaluation based on flagSetId.

It also features “watchers” which enable “listening” for changes within sets of flags returned by arbitrary queries.

In this task, we’ll only be migrating to the mem-db, without adding any new features.