Support multiple backends

[oubiwann]

Probably driven by configuration and/or features ... speaking of which, this depends upon the following:

• 92

Tasks:

• [x] Add support for features
• [x] Move dashmap support to a feature (the default)
• [x] Set persy support as an optional feature
• [x] Set redb support as an optional feature
• [x] Update existing db module to be dependent upon dashmap feature
• [x] Rename dashmap feature to filesystem
• [x] Create a new StoreManager trait withread, write, and backup
• [x] Implement this for the filesystem feature in backend/filesystem.rs
• [x] Add a manager field of type StoreManager to DB
• [x] Cleanup and changes
  • [x] move new into DB implementation
  • [x] move open to same
  • [x] change new to take all the require data for DB (pass, salt, files, etc.)
  • [x] update new to set the StoreManager
  • [x] change open to be a method with &self and use the manager field
  • [x] update close to use the manager field
• [x] Add placeholder modules for persy and redb
• [x] Update backend modules to be dependent upon features

Possibles:

• DashMap (current)
• ~Persy~
• ~Sled~ (buggy; GPLv3 rewrite in progress)
• ~virtual file system solutions~ (see #85)
• redb