gossip-specific cli tool for validator debugging

[mvines]

The --converge-only option solana-bench-tps has been our de facto gossip cli tool for too long, time to level up.

Let's develop a new cli tool, solana-gossip, that includes at least the following functionality:

• Continually monitor gossip and display the contact information for all discovered nodes
• Wait until at least N nodes are discovered
• Wait until exactly N nodes are discovered
• Wait until the node with a given pubkey is discovered

then purge that hacky --converge-only option from solana-bench-tps.

Once complete this new cli will surely impact the content produced by #3261

[mvines]

@CriesofCarrots - I tentatively assigned you for this since I'm AFK for 14 days and I'd love to see this tool in v0.13. But if higher pri stuff comes up in the meantime just de-assign.