cephfs-top: add an option to dump the computed values to stdout
It would be nice if cephfs-top dumps it's computed values to stdout in json format. The json should contain all the fields & corresponding values displayed on the cephfs-top screen for each client, so currently there are 20 fields for a client. When cephfs-top is run with this new option, the ncurses mode would be disabled. This would be really useful if some other module want to access these values exported from cephfs-top. This new option should mainly output the computed values, not just duplicating `ceph fs perf stats` output.