Add Reffered Snapshot Index to Transaction

[GalRogozinski]

Description

Let's define Approved Transaction Root as any confirmed transaction that can be reached by walking from a given transaction down to its parents. The walk must terminate once we reached a confirmed transaction.

We want each unapproved tip to have a field that tells what is the lowest snapshotIndex of its Approved Transaction Root set.

Motivation

We will use this to define a laziness score, see #1771
We want to quickly decide whether we approve a tip that approves a tangle is "Below Max Depth"

Requirements

1. Add a fields to each transaction oldestTxRootSnapshot, youngestTxRootSnapshot
2. Calculate the value of the above fields in O(1) for a new incoming tip assuming that the parents have the value updated.
3. When a new milestone comes in, make sure to update this value for tips in parallel subtangles above the confirmed transactions.

Open Questions (optional)

Perhaps it is best to release this before we are done with the new tipselection?
Whether we should persist those values or not will be handled in a different issue

[GalRogozinski]

Idea: we can start calculating this value only when milestones come in by walking down from milestone until we hit tails with a different snapshot index and then walking up
This won't be O(1) though

[GalRogozinski]

We also need youngest transaction root snapshot. Edited the issue

[GalRogozinski]