Add caching for the Identity COB.

A migration file is added for the sqlite COB cache, which inserts the new identities table.

A new module identity::cache is added, similar to patch::cache and issue::cache.

To achieve the same pattern as patches and issues, it was necessary to introduce a new type Identities in the identity module, which offers the same functionality as methods that existed on the Identity type.

The caching is implemented using write-through semantics, again similar to patches and issues.