Many thanks to @iverberk for the original PR (#1609), but we ended up
not wanting to ship this implementation with 0.5.
We'll come back to it after 0.5 and hopefully find a way to leverage
filesystem accounting and quotas, so we can skip the expensive polling.
* Added the alloc dir move
* Moving allocdirs when starting allocations
* Added the migrate flag to ephemeral disk
* Stopping migration if the allocation doesn't need migration any more
* Added the GetAllocDir method
* refactored code
* Added a test for alloc runner
* Incorporated review comments
The following tests now check a whitelist for whether or not their
driver is present or not, or if the OS is supported or not.
* `TestAllocDir_MountSharedAlloc`
* `TestClient_Drivers_InWhitelist` (`exec` driver)
* `TestClient_Drivers` (`exec` driver)
* `TestJavaDriver_Fingerprint` (`java` driver)
