Storage engine improvements

[MatteoJoliveau]

The current storage engine is a simple abstraction over cloud object storage services such as S3. A file is received by the server and stored as-is in the backing service. This can be inefficient in case of large files.

The following activities can be carried out to improve the storage engine:

• Build an abstraction around the hold crate that can implement custom logic around files and stores metadata in CouchDB
• Split files into equally sized chunks and store those as blobs
• Compress chunks before writing them to reduce storage space