If implemented, will provide users with a way to cache index files.

This addresses issues where the index file is loaded and unmarshalled in
concurrent reconciliation resulting in a heavy memory footprint.

The caching strategy used is cache aside, and the cache is a k/v store
with expiration.

The cache number of entries and ttl for entries are configurable.

The cache is optional and is disabled by default

Signed-off-by: Soule BA <soule@weave.works>

Signed-off-by: Soule BA <soule@weave.works>

Signed-off-by: Soule BA <soule@weave.works>