Avoid null pointer dereferences when dynamic_cast'ing to SwapDir (#743)

Avoid null pointer dereferences when dynamic_cast'ing to SwapDir (#743)

Detected by Coverity. CID 1461158: Null pointer dereferences
(FORWARD_NULL).

When fixing these problems, we moved a few cache_dir iterations into
Disks.cc by applying the "Only Store::Disks can iterate cache_dirs"
design principle to the changed code. The Disks class is responsible for
maintaining (and will eventually encapsulate all the knowledge of) the
set of cache_dirs. Adjusted cache_cf.cc no longer depends on Disk.h.