Re: [IndexedDB] Consider adding a completion event or callback to the IDBDatabase.close() operation (#72)

Thanks for the quick response!

I tried to isolate the simplest test case I could to reproduce the "operation blocked" errors on IE11. It turns out I stumbled upon another unexpected phenomenon, that may or not be related to the particular issue but seems to also happen in Chrome and Edge as well (but interestingly not in Firefox).

The following reduced test case does actually work, but surprisingly both the "blocked", "upgradeneeded" and "success" are triggered on each iteration, on Chrome, Edge and IE11 (in Firefox "blocked" is not triggered but it does succeed):

```js
var lotsOfData = [];

for (var i = 0; i < 10000; i++)
 lotsOfData.push({ prop1: 23523523, prop2: "abcdefghijklmnopqrstuvwxyz"});

var dbName = "bugtesting-" + Date.now();
var version = 1;

function openAndMutateDB() {
 var request = window.indexedDB.open(dbName, version);
 
 request.onerror = function(event) {
  console.log("Iteration " + version + " triggered an error event!");
 };
 
 request.onblocked = function(event) {
  console.log("Iteration " + version + " triggered an operationblocked event!");
 };

 request.onupgradeneeded = function(event) {
  console.log("Iteration " + version + " triggered an onupgradeneeded event!");
  
  var db = event.target.result;
  
  // Create a new object store named as the current version number
  var objectStore = db.createObjectStore(version.toString());
 }
 
 request.onsuccess = function(event) {
  console.log("Iteration " + version + " triggered a success event!");
  
  var db = event.target.result;
  
  // Create a lengthy put transaction and ignore its completion or failure events
  var objectStore = db.transaction(version.toString(), "readwrite").objectStore(version.toString());
  objectStore.put(lotsOfData, "testRecord");
  
  // Abruptly close the connection
  db.close(); 

  // Increment the version variable
  version++;
    
  if (version <= 20)
   openAndMutateDB(); // Continue to reopen the database with a new connection
  else
   window.indexedDB.deleteDatabase(dbName); // Delete the database
 }
}

openAndMutateDB();
```
Removing `db.close();` seems to cause all browsers to only trigger the `blocked` event and stop.

How does this behavior relate to the spec? If "blocked" is triggered, why does the execution continue? Should the event be ignored at this case? (assuming that is even possible?).

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