p2p/simulations: split Network GOD object

[frncmx]

The Network is a GOD object therefore must be split into smaller types.

1. It has 34 public methods ( go doc ./p2p/simulations/.Network | grep func | wc -l) => split
2. Most of the methods locks the whole Network. => refactor pessimistic locking

Example: If a Node is stopped or started the whole network is blocked as we acquire net.lock.

We should split up Network and introduce a more granular locking with smaller critical (lock protected) areas. Currently, Network is hard to modify and it's hard to see "the big picture".