Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo

src/include/optimizer/geqo.h

@@ -22,6 +22,7 @@
 #ifndef GEQO_H
 #define GEQO_H
 
+#include "common/pg_prng.h"
 #include "nodes/pathnodes.h"
 #include "optimizer/geqo_gene.h"
 
@@ -73,7 +74,7 @@ extern double Geqo_seed;		/* 0 .. 1 */
 typedef struct
 {
 	List	   *initial_rels;	/* the base relations we are joining */
-	unsigned short random_state[3]; /* state for pg_erand48() */
+	pg_prng_state random_state; /* PRNG state */
 } GeqoPrivateData;