DAFS: avoid volume lock contention during initialization

DAFS: avoid volume lock contention during initialization

Avoid the excessive volume lock contention during startup to
improve the time to pre-attach a very large number of volumes.
The parallel attach worker threads avoid the volume lock
while scanning the partitions for volumes and send batches of
volume ids to the main thread to be preattached under the
volume lock.

FIXES 124489

Change-Id: Ieb33a3bdd5b06349abd9c3dd994c620021cd6194
Reviewed-on: http://gerrit.openafs.org/1092
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>