Document.atomic context manager

[aogier]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

This PR implements a Document.atomic context manager that don't save a document if there is any uncatched exception in inner code block. Tentatively fixes #427.

Approach

As this change could be breaking (in fact it change a default behaviour) I've decided to implement a brand new context manager using python's contextlib. When it's needed, one could use atomic as a context manager:

with Document(self.db, 'julia006').atomic() as doc:

Context Manager implementation follow current one, in fact it uses the same dunders.

Schema & API Changes

• Added a new Document.atomic context manager

Security and Privacy

• "No change"

Testing

• Added new tests:
  • unit.document_tests.test_document_context_manager_atomic_creation_failure
  • unit.document_tests.test_document_context_manager_atomic_update_failure

Monitoring and Logging

• "No change"

[smithsz]

Thanks again for this. We've decided to change the default behaviour. I've made the change in .__exit__ and stolen your tests (see #434).