[MC-139271] "Preparing spawn area" can log same progress information multiple times

The bug

When loading a world which takes long to load (e.g. because your computer is not that powerful), the "Preparing spawn area" screen logs the same progress information multiple times. This unncessarily increases log files.

19w03c

[22:20:46] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:46] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:47] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:48] [Client thread/INFO]: Preparing spawn area: 0%
[22:20:48] [Client thread/INFO]: Preparing spawn area: 0%