Move next peer selection strategies to a separate file

[devos50]

The strategies to select a peer during the frontier gossip are currently located in the gossip.py. Even though the are an essential part of the gossip strategies, they can be moved to a separate file for a cleaner code organisation.

[Solomon1732]

From what I can see peer selection strategies are done in the following three classes: NextPeerSelectionStrategy, RandomPeerSelectionStrategy, and SmartPeerSelectionStrategy. If this is correct, then they can be moved to a new file. Of course, some tests would have to be updated to match the new locations.

Edit: talking about https://github.com/Tribler/bami/blob/master/src/bami/plexus/backbone/gossip.py