Ticket 49214 - Implement htree concept
Bug Description: Implement the htree concept that was designed
by wibrown.
Fix Description: Implementation of siphash1-3 and htree. The htree
already shows improvements over the b+tree in many conditions,
and a COW implementation will come soon.
https://pagure.io/389-ds-base/issue/49214
Author: wibrown
Review by: mreynolds (Thanks!)