bitswap: keep disconnected peers in backup pool for later reconnection

Ideally, when we disconnect from a peer in a session, we'd reconnect instead of forgetting about them.

A simple approach would be to put them into some form of "backup" pool. If we run out of peers in the session, we'd try connecting to peers in the backup pool. We'd also do this whenever we randomly decide to search the DHT for more peers.

I noticed this issue when trying to bitswap with a local peer. The connection kept getting cut for other unrelated reasons but bitswap wouldn't just reconnect as I'd expect it to.