client: Have hdksize() query cache index rather than cache header
client: Have hdksize() query cache index rather than cache header

- Have hdksize() query disk.idx for the number of valid chunks, rather than
trying to read it out of the cache file header (which no longer contains
that information)

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr