Allow bucket pruning time to be configurable

This was previously hardcoded to 24-hours. This allows the pruning time
to be runtime (and startup) configurable. This can help in cases where
the are frequent partitions, or frequent deletion of buckets.
Fix bug with bucket pruning

riak_cs_utils:fetch_user was returning the wrong boolean for
`KeepDeleted` buckets. This meant that buckets were being pruned in the
opposite scenario from when they should. This change also prunes buckets
even when there are no siblings.
Use better specific specs and documentation

Per review feedback on #812.
Fix microsecond/second mixup bug

This fixes a bug where seconds were used to compare with microseconds.
This would cause deleted buckets to be pruned two orders of magnitude
before they were supposed to.
Fix docstring typo
Merge pull request #812 from basho/bugfix/prune-user-buckets

Bugfix: prune user buckets