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/backend/access/transam/xact.c

@@ -37,6 +37,7 @@
 #include "commands/async.h"
 #include "commands/tablecmds.h"
 #include "commands/trigger.h"
+#include "common/pg_prng.h"
 #include "executor/spi.h"
 #include "libpq/be-fsstubs.h"
 #include "libpq/pqsignal.h"
@@ -1990,7 +1991,7 @@ StartTransaction(void)
 	/* Determine if statements are logged in this transaction */
 	xact_is_sampled = log_xact_sample_rate != 0 &&
 		(log_xact_sample_rate == 1 ||
-		 random() <= log_xact_sample_rate * MAX_RANDOM_VALUE);
+		 pg_prng_double(&pg_global_prng_state) <= log_xact_sample_rate);
 
 	/*
 	 * initialize current transaction state fields