Store/snasphots: add debug events

This simple MR aims to add more logs, at debug level, when importing/exporting snapshots. These logs will be particularly useful in the context of performance regression testing of the snapshot import/export.

Manually testing the MR

Add the TEZOS_LOG=* -> debug flag to a snapshot export or import.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR