Use context-ops abstraction instead of context

What

Remove direct usage of lib_context instead of its abstraction 'context_ops' that will allow for seamless selection of multiple context backends.

Why

To be able to safely and simply select Irmin or Brassaia backends.

How

Manually testing the MR

This MR should not change the behaviour of any existing test nor of any binary of the octez-suite.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR