Fixed bug with PromiseQueue instances sharing same queue.

Fixed issue with Async.PromiseQueue instances sharing the same queue in the prototype.
Fix is to define _queue and _curPromise in the constructor.
Added to Async-test to test for this case.
Applied nitpicks in PromiseQueue shared queue fix.

Set PromiseQueue.prototype._queue to null to avoid confusion.
Removed unneeded _curPromise initialization in constructor.

Signed-off-by: jayther <jayther@gmail.com>
Merge pull request #7485 from jayther/promisequeuesharefix

Fixed bug with PromiseQueue instances sharing same queue.