Weight Random Slicing by storage performance and available space

Ramon Nourequested to merge
rnou/opt2 into master

In MN5 runs, we hit a out of space issue normally in mdtest-hard-write phase. This is a big stopper, a debug analysis show that eventually some of the space is full (due to other users filling the NVMe outside the designed job directory and not deleting the data)

This MR modifies the random slicing calibration (perf) with two weighting (multi ON supported)

* GKFS_RANDOM_SLICING_WEIGHT_SPACE=ON/OFF
* GKFS_RANDOM_SLICING_WEIGHT_PERF=ON/OFF
* GKFS_DISTRIBUTION_STRATEGY=random_slicing
Edited by Ramon Nou

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