Describe the bug
If elFinder.sync() is called while elFinder.sync() is still running, useless backend requests will occur.
This is related to #3568.

To Reproduce
If the user has browser extensions installed that send a Message then this message is intercepted by js/elFinder.js:4827, and since the message is not a string of type JSON an exception is returned and the self.sync() method is called.
Using the example of the wappalyzer add-on - a dozen notifications are sent at once by which elFinder sends multiple requests at once to the application backend.

Expected behavior
New elFinder.sync() execution instructions should be ignored while elFinder.sync() is running.

Screenshots