Flexible peer filtering

[antiochp]

Takes inspiration from #3021 replacing custom cursor impl with impl Iterator.

As part of the PIBD work we will want to run nodes on the network that support various different sync strategies. some nodes may support PIBD while others only support "legacy" txhashset downloads.
So we need more flexible peer management and filtering of connected peers. This PR is a step toward this.

---

Introduce PeersIter so we can return iterator via "newtype iterator delegation".

pub fn peers_iter(&self) -> PeersIter<impl Iterator<Item = Arc<Peer>>> {
    ...
}

Then expose various filters and operators on PeersIter so we can chain them in various flexible ways -

self
    .peers_iter()
    .outbound()
    .connected()
    .count()

---

• moved the random sorting/shuffle out of the api and into a simpler choose_random() filter on PeersIter
• flexible with_difficulty() filtering to filter based on minimum difficulty
• flexible with_capabilities() filtering that can be combined with other chain-able filters

The following fns have all been replaced with the flexible with_difficulty() filter -

• more_work_peers()
• more_or_same_work_peers()
• more_work_peer()
• most_work_peers()
• most_work_peer()

---

Note: As part of this PR the sync process has been modified slightly.
During sync (both header_sync and state_sync) we now sync explicitly against outbound peers.
This makes the sync process more consistent across "public" and "private" nodes. We no longer attempt to sync against inbound nodes regardless of whether we accept incoming connections or not.

[jaspervdm]

Apologies for the delay in review. I really like the approach here 👍 Just have a few small comments