Revert "Replace PostmasterRandom() with a stronger way of generating randomness."

Revert "Replace PostmasterRandom() with a stronger way of generating randomness."

This reverts commit 9e083fd4683294f41544e6d0d72f6e258ff3a77c. That was a
few bricks shy of a load:

* Query cancel stopped working
* Buildfarm member pademelon stopped working, because the box doesn't have
  /dev/urandom nor /dev/random.

This clearly needs some more discussion, and a quite different patch, so
revert for now.