Limit new peer connection rate

[teor2345]

Is your feature request related to a problem? Please describe.

Zebra doesn't limit its outbound connection rate. This is a distributed denial of service risk.

Describe the solution you'd like

• Make CandidateSet.next() into an async function
• Add a timer that yields 1 peer every 0.1 seconds
• Update the docs on new_peer_interval so it's clear that it's the crawl interval, not the peer connection rate limit

Describe alternatives you've considered

This is a critical security issue, so we must do something.

We could limit the connection rate in crawl_and_dial, but that risks a DDoS in any other future users of CandidateSet.

Similarly, we could attempt to limit the connection rate in other parts of the network stack, but that risks future refactors or bugs causing a DDoS.

Additional context

zcashd does not have this issue.