Add stats counter for validate_doc_update rejections

Counting update rejections helps to answer on following questions:
- What is the origin of so anomaly high 401/403 responses?
  We'll be able to distinguish validation errors from regular auth ones.
- Is someone trying to do things we disallowed and how often?
  High rejection rate is a good signal to start figure out do we have
  too strict validation conditions or to start looking for bad clients.
Track the time was spent to execute validate_doc_update function

Validate document update functions are causes significant slowdown on
request processing time when document gets modified. As more validate
functions we have as higher delay will and the couchdb/request_time
metric couldn't tell us why it values are so high and why our service
is so slow. By tracking down the time spent on running all validate
functions for each document update we could see this overhead and decide
just in time if our validation functions are need in some optimizations.