SERVER-120986 introduces OperationSessionTracker which increments the tracked session via getNextSession(). However, ShardingDDLCoordinator and ConfigsvrCoordinator refer to this as getNewSession(). We should consistently use either "new" or "next" to refer to the next session, with my preference being "next."

Additionally, ShardingDDLCoordinator names this function without an underscore, while ConfigsvrCoordinator names it with an underscore. Given ShardingDDLCoordinator uses underscores elsewhere, I'd suggest prefixing all of the session related methods with an underscore as well.