Hash tables backed by DSA shared memory.

Hash tables backed by DSA shared memory.

Add general purpose chaining hash tables for DSA memory.  Unlike
DynaHash in shared memory mode, these hash tables can grow as
required, and cope with being mapped into different addresses in
different backends.

There is a wide range of potential users for such a hash table, though
it's very likely the interface will need to evolve as we come to
understand the needs of different kinds of users.  E.g support for
iterators and incremental resizing is planned for later commits and
the details of the callback signatures are likely to change.

Author: Thomas Munro
Reviewed-By: John Gorman, Andres Freund, Dilip Kumar, Robert Haas
Discussion:
	https://postgr.es/m/CAEepm=3d8o8XdVwYT6O=bHKsKAM2pu2D6sV1S_=4d+jStVCE7w@mail.gmail.com
	https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com