Separate `MemoryStorageClient` and `FilesystemStorageClient`

Description

Currently, we have a MemoryStorageClient, that can persist the data in the file system.

Let's separate them, FilesystemStorageClient could probably extend MemoryStorageClient

Other related things

  • There are memory storage-only data models in the storage/models.py module. Move them to the memory storage subpackage.