feat(p2p): always connected peers

[dignifiedquire]

We should have a set of peers (configurable size) that we store in memory, as main peers. These can be used to send bitswap requests to, as well as improve general connectivity.

[dignifiedquire]

• go-ipfs impl: https://github.com/ipfs/go-ipfs/blob/master/peering/peering.go
• https://docs.ipfs.io/how-to/peering-with-content-providers/#content-provider-list
• AddressBook Attempt in libp2p Add an AddressBook to Swarm libp2p/rust-libp2p#2133
• AddressBook in ipfs-embed https://github.com/ipfs-rust/ipfs-embed/blob/master/src/net/peers.rs

[dignifiedquire]

@mxinden any thoughts/preferences on doing this internal to rust-libp2p vs external? Any plans on reviving sth like an AddressBook again?

[mxinden]

For the record, I will follow up with a design doc incorporating the recent out-of-band discussion.

[mxinden]

@dignifiedquire would you mind following up here?

libp2p/rust-libp2p#2118 (comment)

[mxinden]

I enumerated 3 proposals in libp2p/rust-libp2p#2824.

[mxinden]

For the sake of completeness, latest work by @thomaseizinger is happening here libp2p/rust-libp2p#3099.

[thomaseizinger]

For the sake of completeness, latest work by @thomaseizinger is happening here libp2p/rust-libp2p#3099.

Actually it is libp2p/rust-libp2p#3254.