Currently setFCV and addShard take the fcvLock in exclusive mode so that they don't run together at all.

This ticket must make sure that addShard is safe to run entirely before or after setFCV sets a targetVersion. addShard should be explored to ensure it's safe, and make necessary modifications. One such mod might be to make sure addShard sends a valid FCV when it calls setFCV on the new shard – if in transition, does the function used to acquire a version to put in the call return some non "3.6"/"4.0" string?

shardCollection also takes the fcvLock, specifically for the v3.4 <=> v3.6 transition, due to UUIDs. This will be pulled out eventually (SERVER-33045) but needs to continue working correctly until then. Make sure this doesn't break that either.