rados,mon: improve ceph and rados df reports.

rados,mon: improve ceph and rados df reports.

Fixes: https://tracker.ceph.com/issues/22159
Fixes: https://tracker.ceph.com/issues/20870
This adds  more parameters to estimate allocation granularity overhead and compression
effectiveness. RAW_USED column has been removed. USED column reflects
amount of space allocated for a pool at all replicas. Including
allocaction granularity overhead and taking ompression savings and
object content gaps into account.

Following columns were appended:
* STORED - approximation of bytes users've actually stored in a
pool, i.e netto data amount without compression, allocation and
other overheads (not applicable for 'rados df').
* USED COMPR - amount of bytes allocated to store compressed data, i.e.
compressed data plus allocation/replication overhead.
* UNDER COMPR - amount of data(icluding repicated one) passed through
compression and stored in that form.

Signed-off-by: Igor Fedotov <ifedotov@suse.com>