[Halld-offline] change to cache deletion policy

[Mark Ito]

Folks,

There was a change to how cache files are deleted/unpinned last month. 
The main advance is that files fetched for use by farm jobs that have 
finished get targeted first when looking for stuff to delete. The change 
is not hugely obvious to the user and the explanation highly 
JLab-cache-system dependent, but I thought it should be announced.

Find Ying Chen's email, with some added notes by me at 
https://markito3.wordpress.com/2017/10/10/changes-to-the-cache-deletion-policy/ 
.

   - Mark

-- 
Mark Ito, marki at jlab.org, (757)269-5295