DAFS: Replace partition locks with volume locks

DAFS: Replace partition locks with volume locks

In DAFS, replace uses of the VLockPartition_r partition-level locks with
the approprivate VLockVolume*NB volume-level locks (and sometimes
FSYNC_VerifyCheckout). This allows for greater parallelization of
volserver attachment / volume creation, for volume operations to occur
during salvages, and for multiple salvages on a single partition to
occur simultaneously.

More architectural details of volume-level locks can be found in the
changes to doc/arch/dafs-overview.txt.

Change-Id: I4e8ef4c864002d7e7c976691824c53dfa9cfaf91
Reviewed-on: http://gerrit.openafs.org/1406
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>