Point Parsoid updates to load balancer only

https://gerrit.wikimedia.org/r/#/c/67929/ disables caching in the frontend
caches. This patch avoids duplicate update requests to the backends that are
not coalesced when req.hash_always_miss is true.

Change-Id: I78ce1c241a5962ce4a3481e2081308311b4d1283
Merge "Point Parsoid updates to load balancer only"