Improve setFeatureCompatibilityVersion 4.2 impact with many tables

When a primary executes setFCV("4.2") its main work is to upgrade all its unique indexes to a new format (SERVER-32825). It writes a collMod oplog entry per collection to trigger the secondaries to upgrade their unique indexes too.

When mongos executes setFCV, it sends it to the primary of the config replica set, which sends it to the primary of each shard, and waits for majority acknowledgement.