Windows: improve store data parallelism

Windows: improve store data parallelism

The file server will set the rx call status bit (0x1)
when the rpc is in process and all of the locks are held.
At this point it is not possible for another store data rpc
to begin on the vnode prior to the completion of the current
rpc.  Once this status bit is detected as set, the exclusive
store data synchronization on the cm_scache_t can be dropped.

This permits the next store data rpc to perform its biod
construction.

Change-Id: Ic856769650781b4f5f4ab4ac86df4946496bd655
Reviewed-on: http://gerrit.openafs.org/5741
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>