MB-9434: prevent deadlock between flusher & stats

If the memcached stats thread which is holding the thread
lock tries to acquire the high priority checkpoint list
lock just to read its size, it can deadlock on the
flusher thread which may be holding the list lock & trying
to notify the same memcached thread doing the stats

To avoid this deadlock maintain the size of the high
priority checkpoints in a separate variable and allow for
dirty read on the size

Change-Id: I6f0991c22deb2cc7762e5967496b99c3baed2c12
Reviewed-on: http://review.couchbase.org/29900
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>