Fix for WalrusImageCache failure to cache after size changed. Fixes EUCA-7269.

Includes optimizations for fast-failure for images that will not fit in the cache to
avoid flushing the cache or doing a part assembly if the image will never fit.

Also includes a new configurable property: cloud.images.maximagesizegb that gives the
admin explicit control over the maximum image size that is allowed to be registered. The
check is at registration time and will return an error to the user if the image is too large.
The default for new installations is 30GB (same as walrus cache size default) and for upgrade
the value is set to 0 (<=0 is treated as unlimited size on registration check).

Commit also includes a lot of logging changes to present the behavior of the cache to the admin.
Log entries are prefixed with [CorrelationId: <id> | Account: <account> ] for cache requests and
unbundling/caching tasks are prefixed with [Caching Task: <task thread id>].