ACCUMULO-3393 Follow-on work for per-table volume chooser.

* docs clean up + code guideline compliance.
* ensure RandomVolumeChoosers are independent when used per-table.
* make sure that per-table choosers can keep state the way that global choosers can
* make sure that a chooser can only pick from the options it is presented.
* reduce object creation in critical path for Tablet.split.