Manage EBS volumes automatically as blockdevs

[agrover]

see https://lwn.net/Articles/755948/

I think auto-extending the EBS volume when the pool approaches capacity is a cool first thing we could consider. APIs ftw!

The comment also alludes to removing the volume when it's no longer needed... this would be a little bit harder. Right now, once a blockdev has thinpool data dev extended onto it, it doesn't ever move or shrink.

[ibotty]

Maybe the same could be done with RBD-Devices (Ceph block devices).