Support more collapsed forwarding hit cases:

Support more collapsed forwarding hit cases:

Allow STORE_MEMORY_CLIENTs to open disk files if needed and possible.
STORE_*_CLIENT designation is rather buggy (several known XXXs). Some
collapsed clients are marked as STORE_MEMORY_CLIENTs (for the lack of info at
determination time) but their hit content may actually come from a disk cache.

Do not abandon writing a collapsed cache entry when we cannot cache the entry
in RAM if the entry can be cached on disk instead. Both shared memory cache
and the disk cache have to refuse to cache the entry for it to become
non-collapsible. This dual refusal is difficult to detect because each cache
may make the caching decision at different times. Added StoreEntry methods to
track those decisions and react to them.

Recognize disk cache as a potential source of the collapsed entry when the
memory cache is configured. While collapsed entries would normally be found in
the shared memory cache, caching policies and other factors may prohibit
memory caching but still allow disk caching. Memory cache is still preferred.