rx: Make statistics interface use Atomics

rx: Make statistics interface use Atomics

Make the rx_statistics statistics gathering infrastructure use
atomics for all of its counters. This significantly reduces
lock contention. However, it also (potentially) changes the format
of the rx_stats variable which has been used by callers in the past.

To simplify this process, and to aid with future changes, we remove
direct access to rx_stats. Instead, two additional API functions
rx_GetStatistics and rx_FreeStatistics are provided. These give the
caller access to an 'normal' rx_statistics structure.

Tom Keiser has suggested that we should explore using thread-local
statistics structures, and just aggregating them when we are asked
to report. This is a fine idea, and is equally possible with the
new interface that this patch introduces.

Change-Id: I859cea8f7354a655be007b95fa8a61f995308b35
Reviewed-on: http://gerrit.openafs.org/2862
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>