Phase Out Legacy Network Stack Gradually

[aszepieniec]

The libp2p stack has been operational for a few months now. The reason for keeping the legacy network stack was to have a spare leg to stand on in case a vulnerability in libp2p is discovered. However, that spare leg also comes with extra attack surface -- cancelling the argument. In the end, the prettier solution is the one that involves fewer lines of code.

This task can be instantiated in many ways but here is one possibility:

• disable legacy peer discovery unless with active CLI argument
• disable remainder of legacy stack unless with active CLI argument
• disable entire legacy stack unless with compilation flag
• remove legacy network stack code entirely