Blockdev types and special handling

[agrover]

As mentioned in #1246, there are many blockdevs that may have special capabilities. Some (all?) of these have APIs. It would be cool if a Stratis pool on such a blockdev could request more space from the underlying provider when the pool approached its full capacity. For example, see https://aws.amazon.com/ebs/pricing/ . Users could save money by creating a pool on a small EBS device, and then Stratis would be smart enough to request additional space using the EBS API as the pool filled up.