Introduce the structures for incremental index for format spec 2.5

[piste-jp]

Summary of changes

Implement the data structure for incremental index

Description

The incremental index is introduced into LTFS format spec 2.5. But nobody tried to implement it. So I decided to do it.

This PR is the first step to support the incremental index. Just record the changes against filesystem as inc_journal and dump if by ltfs.vendor.IBM.dumpincj VEA.

The inc-journal will be cleared when

1. ltfs.vendor.IBM.dumpincj is accessed
2. Full index is generated in any reason

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have confirmed my fix is effective or that my feature works

[piste-jp]

@juliocelon ,

Could you merge this as soon as possible? I would like to make other changes on this PR in the next week.