spacemanager: Avoid aggressive error output on cell communication

Without this patch we may quickly fill the log file with the same
error message about cell communication failure.

The patch also tries to distinguish between transient and non-transient
database failures when expiring files. Both are retried, but non-transient
failures are not retried until the expiration period has passed
(usually 3 minutes).

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6488/