log size of assets when found in cache
log size of assets when found in cache

[#156440748](https://www.pivotaltracker.com/story/show/156440748)

Signed-off-by: Nima Kaviani <0ebde517669f70c2aec92dc89d4950413b825ec0@us.ibm.com>

cloudfoundry-incubator/cacheddownloader