Mongod mergeChunks command already uses an optional 'epoch' command field. This is basically a line or two addition to the cluster command to include the field.

For v3.8 we can make the mongod mergeChunks command require the 'epoch' field, making it no longer optional. We cannot do this is v3.6 because we must maintain backwards compatibility with a v3.4 mongos not sending the 'epoch' field.