Change active config from file storage to DB storage

Problem/Motivation

1. During a cold start, we load a lot of config files from disk. These files must be stored on the shared filesystem when using multiple web heads. This is slow and often unreliable.
2. We have an active config directory which looks editable but if you actually dare to edit it, things will break horribly. This is unnecessary brittleness.

Proposed resolution

1. We retain the pluggable storage backend support for active configuration. Core ships with db storage by default, with no caching.
2. Core also ships with a db-backed storage with a cache layer. This will allow sites with memcached/redis support to take advantage of their existing non-db cache layer for config without writing a new storage driver. They can simply configure Drupal to use the cache-layer storage with memcached/redis as the cache backend.

Remaining tasks

Decide on a serialization format in the DB.

User interface changes

None.

API changes

None.