Fix: Ensure proper creation of solid entrypoints

[GalRogozinski]

Description

Enforces the following definition for a solid entry point:

A transaction is considered a solid entry point if it is a bundle tail that can be traversed down from a non-orphaned transaction that was approved by a milestone that is above the last local snapshot. Or if it is a bundle tail of a non-orphaned transaction that was approved by a milestone that is above the last local snapshot.

Before non-tails could have been solid entry point, this caused problems in the case they were common to different bundles.

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How Has This Been Tested?

• My node was able to sync with the LS created.
• Using https://github.com/iotaledger/Snapshot.ixi I compared the ledger state of a node that is synced from last global snapshot to my synced node.

Checklist:

• 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[GalRogozinski]

We may want to release this quick as a hotfix.
My manual tests should suffice. A unit test can be created later...

Also this is not the most efficient implementation regarding db queries, but this can also be tended to later.

[GalRogozinski]

After enough testing this will be merged w/o review in order to advance the release of the hotfix...
Can be later changed with subsequent PRs

[acha-bill]

Looks good