MB-27769: Add the map of vbuckets in the CheckpointProcessorTask to stats
MB-27769: Add the map of vbuckets in the CheckpointProcessorTask to stats

CheckpointProcessorTask contains the queue of vbuckets and a map to
indicate which vbuckets are already present in the queue.

We currently only track the contents of the queue in stats. This commit
also adds the contents of the map to the stats.

Change-Id: I8d0c6df876c37aeb8b69275925562c4c842ce784
Reviewed-on: http://review.couchbase.org/89129
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine