Windows: Track active RPCs per scache_t

Windows: Track active RPCs per scache_t

It has been noticed that multiple RPCs can be active on
a cm_scache_t object at the same time.  This is especially
true of directory objects with the redirector.  Track the
number of active RPCs and use that number in cm_MergeStatus
when deciding whether or not to discard the cached data for
the object.

Change-Id: If291bb4c0e48d0ec087c3a7c2640e4598e7fd419
Reviewed-on: http://gerrit.openafs.org/6001
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>