Assign Scores To Tips

[GalRogozinski]

Note
The feature request will probably be integrated faster if you do a pull request for it.
If you want to discuss the feature before you actually write the code you are welcome to do it by first submitting an issue.

Description

Part of https://github.com/GalRogozinski/protocol-rfcs/blob/be73f3fe0bdb0fef4395401a1acc06293d878398/text/0004-new-pre-coordicide-consensus/0004-new-pre-coordicide-consensus.md

We want to assign a score weights on the tips the nodes sees. The score will signify how lazy the transaction is.
0 will be given to lazy tips
1 will be given to somewhat lazy tips
2 will be given to non-lazy tips

Motivation

The score will be used by our tip-selection.

Requirements

1. Implement milestone proposal from the RFC#4
2. The score must only be saved in memory for solid tips. Persisting will be for another issue

FAQ

What is the meaning of a lazy tip?

non formal: lazy transactions have connection that are far in the back. We want to promote them.

What attacks are we preventing?

We just want to demotivate lazy behavior and have a heuristic for below max depth

Do we want to prevent sparse tangles?

It happens automatically since 1 tx approves 2

Do we want to mimick local modifiers?

a bit

What will happen in a tangle where some nodes will use old tip-sel and some new?

Shouldn't be a big deal

[GalRogozinski]

We need to notice, that invalid tips will have 0 score as well