Manage pNFS Data Servers

* Parallels export manager.
* Moved DS handle creation to pDS ops vector (and *ds.c files).
* Added FSAL op for creation of subsequent pDS ops vectors.
* Added pDS op for creation of subsequent DS handle ops vectors.

+ FSAL_CEPH with minimal support, requires DS {} configuration.
+ FSAL_GPFS manages DS {} at same time as EXPORT {}.
+ FSAL_LUSTRE manages DS {} at same time as EXPORT {}.

- Merged static one time use functions in FSAL/commonlib.c
- Provided default pDS creation/destruction in FSAL/default_methods.c
  (unlike exports).
- Increase refcount size to 64 bits for DS handles, matching others.
- Related minor field/function renaming.

Bug fix: in PutFH, put before get, then comparing old id, could cause
use of dereferenced data.  Always get (inc) new before put (dec) old.

Bug fix: in PutFH, fh_desc.addr should have pointer to data
(match DS side with export side).

Bug fix: wrong macro list element in shutdown_pnfs_ds() --
motivating field rename.

Signed-off-by: William Allen Simpson <bill@cohortfs.com>