Feat: Making IRI use the initialSnapshot and the solidEntryPoints

[hmoog]

Description

This PR contains the changes that are necessary for checking against the solidEntryPoints instead of the NULL_HASH. It does not activate the creation of local snapshots, yet and also doesn't modify the LedgerValidator and MIlestoneTracker to change the "latestSnapshot" instance. The "initialSnapshot" that holds the solidEntryPoints is however read already from the new SnapshotProvider so this activates parts of the new code already. Since the "initialSnapshot" will be the last global snapshot (with the NULL_HASH as its only solidEntryPoint) in the absence of local snapshot files, it is a non-breaking change.

Once this PR is merged there will be a last PR that also enables the use of the "latestSnapshot" instance, which will fully enable the new features.

Type of change

• Enhancement (a non-breaking change which adds functionality)

How Has This Been Tested?

• All integration tests should succeed

Checklist:

Please delete items that are not relevant.

• My code follows the style guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

[GalRogozinski]

Just fix the little typo and we will merge