Use new ethers.js NonceManager to handle transaction nonces

So far we have failed to globally handle the transaction nonces.
The new ethers.js v5 comes with a NonceManager that helps us handling
transaction nonces and automatically increases the nonce for each
transaction.

package.json

@@ -12,7 +12,8 @@
   "dependencies": {
     "cors": "^2.8.5",
     "eth-provider": "^0.2.2",
-    "ethers": "^4.0.27",
+    "ethers": "^5.0.2",
+    "@ethersproject/experimental": "5.0.0",
     "express": "^4.17.1",
     "express-session": "^1.16.2",
     "grant-express": "^4.6.1",