Initial work on having DiskLayer implement the full StoreReadLayer API.

This introduces mainly a set of unsupported operations into DiskLayer, and
introduces a hidden config option to disable the cache layer entirely. Some
operations, like nodeExists and relExists, have been implemented.

This would be a first step for assessing database performance
without the object cache.
Merge pull request #2752 from jakewins/2.2-disk

Initial work on having DiskLayer implement the full StoreReadLayer API.