benchmark: Add a createInDir var in BenchmarkMgr

Together with its setters/getters.
benchmark: Make threads use their own folder if set

It depends on the createInDir variable of the BenchmarkMgr.
benchmark: Fix small cosmetic issue in createFiles
benchmark: Make the createInDir flag settable from CL args
benchmark: Add help about the "create in dir" option
benchmark: Show info about threads using their own dir
Fix the entries' prefix when compacting a dir

It was not adding the + before the name of the entry. It was not
a problem, but having it might avoid bugs in the future.
test: Add coverage for compacting a dir with metadata