In FileSystemRMStateStore, we can refine some fs operations to improve performance:
1. There are several places invoke fs.exists, then fs.getFileStatus, we can merge them to save one RPC call
The updateFile is not good too, it write file to output_file.tmp, then rename to output_file.new, then rename it to output_file, we can reduce one rename operation.
Also there is one unnecessary import, we can remove it.