Move syncer block download retries inside the peer set

[teor2345]

Motivation

In PR #3191, we add a yield to syncer block download retries, so that:

• we're more likely to choose a different peer, and
• any notfound inv entries win the race to the PeerSet

But if we did block download retries within the PeerSet, we could always choose different peers. This would also help us limit retries to the number of available peers.

This prevents a single failing peer from monopolising all peer requests.

Designs

Move the inner block download RetryLimit(2) from the syncer to the PeerSet:

• choose different peers, if we have at least two available
• mark them both unready
• send the request to the first peer
• if the first request fails, send the request to the second peer
• if the first request succeeds, cancel the second peer and make it ready again (needs tests)

After this change, gossiped blocks would also get retries, but that's probably ok.

Related Work

• Security: Send notfound inv items to the inv collector, Credit: Equilibrium #2156

[teor2345]

This is obsoleted by the notfound changes in #2156