feat: async peerstore backed by datastores (#1058)

We have a peerstore that keeps all data for all observed peers in memory with no eviction.

This is fine when you don't discover many peers but when using the DHT you encounter a significant number of peers so our peer storage grows and grows over time.

We have a persistent peer store, but it just periodically writes peers into the datastore to be read at startup, still keeping them in memory.

It also means a restart doesn't give you any temporary reprieve from the memory leak as the previously observed peer data is read into memory at startup.

This change refactors the peerstore to use a datastore by default, reading and writing peer info as it arrives.  It can be configured with a MemoryDatastore if desired.

It was necessary to change the peerstore and *book interfaces to be asynchronous since the datastore api is asynchronous.

BREAKING CHANGE: `libp2p.handle`, `libp2p.registrar.register` and the peerstore methods have become async

test/peer-routing/peer-routing.node.js

@@ -658,10 +658,8 @@ describe('peer-routing', () => {
 
       await node.start()
 
-      await delay(300)
-      expect(node._dht.getClosestPeers.callCount).to.eql(1)
-      await delay(500)
-      expect(node._dht.getClosestPeers.callCount).to.eql(2)
+      // should run more than once
+      await pWaitFor(() => node._dht.getClosestPeers.callCount === 2)
     })
   })
 })