don't clear challengeCallbacks

src/certnode/lib/client.js

@@ -445,7 +445,8 @@ class Client {
           return challenge.token + '.' + this.thumbprint;
         hasResolved = true;
         clearTimeout(time);
-        this.challengeCallbacks = null;
+        // wanted to clear callbacks here but LE does the call multiple times.
+        // remember we're in mutex lock so no worries for racing.
         return challenge.token + '.' + this.thumbprint;
       }
     });

src/sni.js

@@ -89,6 +89,8 @@ async function getKeyCert(servername) {
 let lock = new AwaitLock();
 
 const SniListener = async (servername, ctx) => {
+    // Had to use lock because the best authenticator
+    // library seems don't yet fully stateless.
     // Generate fresh account keys for Let's Encrypt
     await lock.acquireAsync();
     try {