dcache: Do not cache gPlazma timeouts

The gPlazma cache used by NFS and SRM caches both positive and negative
replies. Unfortunately it also caches failures to communicate with gPlazma.
This means that a transient timeout would be cached too, thus increasing the
effect of the transient error.

This patch resolves this problem by not caching timeouts (which include failure
to communicate with gPlazma).

Other errors are still cached - the correctness of this is debatable when
gPlazma does not distinguish between transient and permanent errors.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7332/