Fix the tar snapshot generation and bump them to version 7

Context

This MR fixes the issue that produced invalid tar snapshots which were corrupted when the context file was bigger than 8GB due to a false assumption that tar headers are always 512B: when the tar-ed files are bigger, the Tar library automatically bump their size to 512Bx3 which made us overwrite data with some extended tar header leading in a corrupted archive.

This MR also bump snapshots version from 6 to 7 and adds a warning message whenever users try to import a v6 (or below) tar snapshot.

Manually testing the MR

• Export & Import a v7 snapshot using this version
• Import a v5/v6 tar (also raw) snapshot (corrupted and valid): it should display a warning message when in importing a tar snapshot.

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