humanize.Time() drops precision resulting in some events reporting they took
"now" time to complete. Using time.Duration.String() results in accurate
duration being reported.
Fixes#13905
When performing the downgrade operation, users can confirm whether each member
is ready to be downgraded using the field 'storageVersion'. If it's equal to the
'target version' in the downgrade command, then it's ready to be downgraded;
otherwise, the etcd member is still in progress of processing the db file.
