Update RealtimePlumberSchool.java

There was an issue if you restart realtime node and merged dir is still there it will throw exceptions. To tackle this issue a check has been added. So now only persisted dir/files will be read and anything from "merged" will be avoided.
Update RealtimePlumberSchool.java

Put a check to make sure that we're only looking for the folder those have number-only names, to avoid "merged" directory issue.
Merge pull request #158 from kOD-dKOD/master

Tackle "merged" dir. issue after restarting realtime node