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/utils/creators/peer.js

@@ -58,12 +58,13 @@ function _populateAddressBooks (peers) {
  * @param {Object} [properties]
  * @param {number} [properties.number] - number of peers (default: 1).
  * @param {boolean} [properties.fixture] - use fixture for peer-id generation (default: true)
+ * @param {PeerId.CreateOptions} [properties.opts]
  * @returns {Promise<Array<PeerId>>}
  */
-function createPeerId ({ number = 1, fixture = true } = {}) {
+function createPeerId ({ number = 1, fixture = true, opts = {} } = {}) {
   return pTimes(number, (i) => fixture
     ? PeerId.createFromJSON(Peers[i])
-    : PeerId.create()
+    : PeerId.create(opts)
   )
 }