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/libp2p-in-the-browser/.babelrc

@@ -1,4 +1,3 @@
 {
-  "presets": ["@babel/preset-env"],
   "plugins": ["syntax-async-functions","transform-regenerator"]
 }

examples/libp2p-in-the-browser/package.json

@@ -14,12 +14,11 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
-    "@babel/preset-env": "^7.13.0",
+    "@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-star": "^0.23.0",
+    "libp2p-webrtc-star": "^0.25.0",
     "libp2p-websockets": "^0.16.1"
   },
   "devDependencies": {