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

examples/webrtc-direct/listener.js

@@ -40,5 +40,4 @@ const PeerId = require('peer-id')
 
   console.log('Listening on:')
   node.multiaddrs.forEach((ma) => console.log(`${ma.toString()}/p2p/${node.peerId.toB58String()}`))
-
 })()

examples/webrtc-direct/package.json

@@ -20,10 +20,10 @@
     "util": "^0.12.3"
   },
   "dependencies": {
+    "@chainsafe/libp2p-noise": "^5.0.2",
     "libp2p": "../../",
-    "libp2p-bootstrap": "^0.13.0",
+    "libp2p-bootstrap": "^0.14.0",
     "libp2p-mplex": "^0.10.4",
-    "@chainsafe/libp2p-noise": "^4.1.0",
     "libp2p-webrtc-direct": "^0.7.0",
     "peer-id": "^0.16.0"
   },