Move query server related stats from couch to couch_mrview

A part of https://github.com/apache/couchdb-couch/pull/9
The very equivalent place is this one anonymous function https://github.com/apache/couchdb-couch-mrview/blob/master/src/couch_mrview_updater.erl#L150 , but I'm more not sure if it's good place for stats handling.