Update 2019-02-07: Design decision is that mongodump/mongorestore will not support backing up individual shards in a 4.2 sharded cluster due to consistency limitations. Specific changes to make:

• Document this limitation
• Skip dumping the config.transaction_coordinators collection

Update 2019-02-22: Need for large transaction support (TOOLS-2225) will require similar behavior to prepared transaction support: namely, "on commit, look back in oplog to apply operations". Therefore, we might reconsider just doing prepared transactions support at the same time as the incremental work will be small.