[breaking] Refactor Python user API

[cbornet]

This is a breaking change of the API but it really simplifies usage for the user.
It also brings the possibility to return the process/write results asynchronously.

Changes (see api.py and util.py)

• Sink::write takes a single record (previously a list)
• Remove Sink::set_commit_callback and CommitCallback : now a successful return of the write method commits the record. It's also possible to return a Future and the commit will happen when the future completes.
• Processor::process takes a single record (identical to SingleRecordProcessor)
• Processor::process can return a list of records or a future that will complete with the list of records.
• Remove SingleRecordProcessor

[eolivelli]

Please rebase onto the main branch the other branch has been merged