try to fix the "Query after fatal error" problem

src/db/devicemessages.js

@@ -1,13 +1,23 @@
 //devicemessages
 var mysql      = require('mysql');
-var con = mysql.createConnection({
+var pool  = mysql.createPool({
+  connectionLimit : 20,
   host     : 'localhost',
   user     : 'iot',
   password : '!iot_popovi',
   database : 'iot'
 });
+// var con = mysql.createConnection({
+//   host     : 'localhost',
+//   user     : 'iot',
+//   password : '!iot_popovi',
+//   database : 'iot'
+// });
 
 //const got = require('got');
+pool.on('acquire', function (connection) {
+  console.log('Connection %d acquired', connection.threadId);
+});
 
 exports.findByName = function(fieldName, days, cb) {
   process.nextTick(function() {
@@ -29,15 +39,19 @@ exports.insert = function(device_id, field_name, value, cb){
     var params = [device_id, field_name,value];
     let sql = `INSERT INTO devicemessages(device_id,field_name,field_value,timestamp)
     VALUES (?,?,?,NOW());`;
-    con.query(sql,params,(err, r) => {
-      //if(!cb) {return;}
-      if (err) {
-        console.log("error: ", err);
-        cb && cb(new Error('SQL Error: ' + err));
-      }else{
-        console.log("inserted record: ", { id:  r.insertId});
-        cb && cb(null, { id: r.insertId, ...params });
-      }
+    pool.getConnection(function(err, conn) {
+      if (err) throw err; // not connected!
+      con.query(sql,params,(err, r) => {
+        con.release();
+        //if(!cb) {return;}
+        if (err) {
+          console.log("error: ", err);
+          cb && cb(new Error('SQL Error: ' + err));
+        }else{
+          console.log("inserted record: ", { id:  r.insertId});
+          cb && cb(null, { id: r.insertId, ...params });
+        }
+      });
     });
 }
 exports.getFromDht = function(url, cb)
@@ -56,4 +70,4 @@ exports.getFromDht = function(url, cb)
         cb && cb(new Error('SQL Error: ' + ex));
       }
     })();
-}
+};