simplehash: Add support for resetting a hashtable's contents.

A hashtable reset just reset the hashtable entries, but does not free
memory.

Author: Andres Freund
Discussion: https://postgr.es/m/20190114180423.ywhdg2iagzvh43we@alap3.anarazel.de
Bug: #15592 #15486
Backpatch: 11, this is a prerequisite for other fixes