libafs: vldb cache timeout option (-volume-ttl)

libafs: vldb cache timeout option (-volume-ttl)

The unix cache manager caches VLDB information for read-only volumes as
long as a volume callback is held for a read-only volume.  The volume
callback may be held as long as files in the read-only volume are being
accessed.  The cache manager caches VLDB information for read/write
volumes as long as volume level errors (such as VMOVED) are not returned
by a fileserver while accessing files within the volume.

Add a new option to set the maximum amount of time VLDB information will
be cached, even if a callback is still held for a read-only volume, or
no volume errors have been encounted while accessing files in read/write
volumes.

This avoids situations where the vldb information is cached indefinitely
for read-only and read/write volumes.  Instead, the VL servers will be
periodically probed for volume information.

Change-Id: I5f2a57cdaf5cbe7b1bc0440ed6408226cc988fed
Reviewed-on: https://gerrit.openafs.org/11898
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>