Validate document update stats

This adds two new metrics about counting validation rejections and the time was spent on running vdu functions. The reason for each inside commit messages.
Not sure I follow, but do you suggest to add another case before `case Resp of` like:
```
case Resp of
  _ when Resp /= 1 -> couch_stats:increment_counter([couchdb, query_server, vdu_rejects], 1)
end,
```