Implement no-cleanup options and improve file exchange

Implement the no-cleanup options from the xtfs_benchmark tool.
Improve the file exchange between benchmarks, now list of files are exchanged
between all benchmarks. This allows for better reuse of previously written files
in read-only benchmarks and allows for deleting files without deleting the volume.
Note: There seems to be a bug in the Volume.readDir method which prevents getting
filelists greater than 1024 files.