VMware: image cache aging

The current patch set deletes cached images that are no longer used.
Images that are unused will be marked for aging. These images
will be deleted after they have not been used for
'remove_unused_original_minimum_age_seconds'.

The third and final phase of blueprint vmware-image-cache-management.

NOTE: in order to ensure that there will not be race conditions with
multiple compute nodes, each compute node will have its own datastore
cache directory. This will be a combination of the configuration
variables my_ip and image_cache_subdirectory_name. If the aging is
disabled then the original base is used (ensures backwards
compatibility).

Locking is done when accessing timestamps to detect if an image has aged
in the cache.

Change-Id: Iec47c28a38761c187226c5eff3ab69da503437f6
Merge "VMware: image cache aging"