adding tornado

we need this to run asyncrhonous command inside the main ioloop

backports is a dependecy of tornado
imgstorage: use tornado IOLoop

we use now the tornado ioloop to manage pika request

Nothing changes in theory since the two ioloop has the same interface
In practice we had to do some changes to properly shuts down the
ioloop when we stop the server.
imgstorage: make popen asynchronous inside the IOLoop

This patch make most of the popen (fork to run another command)
asynchronous, so they don't block the main ioloop.
To do this we used tornado coroutines (tried to do it by my self,
but it was too complex) which make asynchronous call transparent
to the caller (you just need to use yield keyword)

imgstoragevm is not tested yet with this patch....
imgstorage: check before unmapping block device device - take II

So the fuser is useless, file is not owned by anybody but the call
still fails with:
remove ioctl on xxxxxx failed: Device or resource busy

So let's try if
dmsetup remove --retry
works