v2+extindex: show commit time and indexing rate in progress

v2+extindex: show commit time and indexing rate in progress

With the `-v' switch, we'll display these rates to track total
indexing rate and commit speeds throughout the indexing phase.
These numbers will help us monitor for slowdowns throughout the
entirety of a large indexing job taking several days.  This
change may help us decide whether or not to start implementing
autodefrag for btrfs and similar CoW FSes prone to performance
degradation from fragmentation.