Command save-off does not disable periodical autosaving

In servers, save-off does not disable autosaving and autosaves continue to happen at 5 minute intervals. This is catastrophic for backup scripts which have no choice but to rely on the save-off functionality to capture a consistent world state.

Steps to reproduce:

1. Launch a new Minecraft server and wait for the world to generate
2. Run save-off in the console and note the time
3. Wait 5 minutes
4. Notice that the file modify time for world/level.dat is a time after the time save-off was run and continues to be updated every 5 minutes

This is different from MC-4417 because the autosave continues happening, even occurring multiple times after the save-off command is run. This is not simply an issue of waiting for a single save to finish.

This is different from MC-3208 because this has nothing to do with the /stop command.