Move statistics and and call-closing from afs_segments to afs_fetchstore.

Move statistics and and call-closing from afs_segments to afs_fetchstore.

Rather than in afs_StoreAllSegments, do statistics in afs_CacheStoreProc,
further boosting the former's readability. Calls are closed in a new
storeOp now, rxfs_storeClose, and have been moved there from
afs_StoreAllSegments as well.

Reviewed-on: http://gerrit.openafs.org/117
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>