Calculate Entry Point without COO and use Katz centrality

[stplaydog]

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

Current IOTA use entryPointSelector.getEntryPoint to calculate the entry point, and this method simply use the COO as the entry point. This leads to the centralized assumption of IOTA. In StreamNet, we should avoid using COO.
Instead we will use Katz centrality to decide the entry point, this method is purely mathematical.

Motivation

Avoid centralized DAG.

Requirements

Will give the entry point properly.
Will have a metric for evaluation.

Open Questions (optional)

Am I planning to do it myself with a PR?

Yes.