* Refactored disk-based key-value pair read and write operations from DiskCacheService to DiskServiceMixin.
* Refactored DiskCacheService to mixin DiskServiceMixin.
* Removed Sqlite3StoreService and Sqlite3StoreProvider implementations.
* Added DiskStoreService and DiskStoreProvider implementations.
* Removed stub MemoryMessageService and MemoryMessageProvider.