Prevent reads during the merge of the prepared world changes.

This prevents issues when a chunk/player gets loaded while the file they
are reading from gets replaced.

For example this race condition could lead to this crash:
https://gist.github.com/Cervator/0f959ed839c5d1463f0f#file-random-failure-to-load-a-chunk

For details have a look at the javadoc of:

STorageManagerInternal#worldDirectoryLock