p2p: improve peerStore prototype (#5954)

This improves the `peerStore` prototype by e.g.:

* Using a database with Protobuf for persistence, but also keeping full peer set in memory for performance.
* Simplifying the API, by taking/returning struct copies for safety, and removing errors for in-memory operations.
* Caching the ranked peer set, as a temporary solution until a better data structure is implemented.
* Adding `PeerManagerOptions.MaxPeers` and pruning the peer store (based on rank) when it's full.
* Rewriting `PeerAddress` to be independent of `url.URL`, normalizing it and tightening semantics.

node/node.go

@@ -762,7 +762,11 @@ func NewNode(config *cfg.Config,
 	logNodeStartupInfo(state, pubKey, logger, consensusLogger)
 
 	// TODO: Fetch and provide real options and do proper p2p bootstrapping.
-	peerMgr := p2p.NewPeerManager(p2p.PeerManagerOptions{})
+	// TODO: Use a persistent peer database.
+	peerMgr, err := p2p.NewPeerManager(dbm.NewMemDB(), p2p.PeerManagerOptions{})
+	if err != nil {
+		return nil, err
+	}
 
 	csMetrics, p2pMetrics, memplMetrics, smMetrics := metricsProvider(genDoc.ChainID)
 	mpReactorShim, mpReactor, mempool := createMempoolReactor(config, proxyApp, state, memplMetrics, peerMgr, logger)