Export resulting context hash in snapshots

This MR aims to fix the (* FIXME: provide the resulting block context in the snapshot and check against it when the [expect_predecessor_context] semantics is used. *) comment by exporting the context hash resulting from the application of the imported block of a snapshot.

Additionally it:

• (minor) refactors snapshot version and header read
• removes the support of v2 and v3 legacy snapshots

Manually testing the MR

Check that the legacy snapshots can still be imported.

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