Cache: allow configuring disk usage low/high water marks

[matyasselmeci]

Allow configuring the disk usage high water mark/low water mark since the appropriate values vary depending on the speed of the storage medium. The high water mark is the amount of disk used before the cache starts deleting files; the low water mark is the amount of disk used below which the cache stops deleting files.

This is the directive pfc.diskusage <LOW> <HIGH> listed in the xrootd proxy configuration doc; LOW and HIGH are either a fraction of 1.0, or an absolute value suffixed with k, g, m, or t. The default value is

pfc.diskusage 0.90 0.95