Switch to CRC-32C in WAL and other places.

The old algorithm was found to not be the usual CRC-32 algorithm, used by
Ethernet et al. We were using a non-reflected lookup table with code meant
for a reflected lookup table. That's a strange combination that AFAICS does
not correspond to any bit-wise CRC calculation, which makes it difficult to
reason about its properties. Although it has worked well in practice, seems
safer to use a well-known algorithm.

Since we're changing the algorithm anyway, we might as well choose a
different polynomial. The Castagnoli polynomial has better error-correcting
properties than the traditional CRC-32 polynomial, even if we had
implemented it correctly. Another reason for picking that is that some new
CPUs have hardware support for calculating CRC-32C, but not CRC-32, let
alone our strange variant of it. This patch doesn't add any support for such
hardware, but a future patch could now do that.

The old algorithm is kept around for tsquery and pg_trgm, which use the
values in indexes that need to remain compatible so that pg_upgrade works.
While we're at it, share the old lookup table for CRC-32 calculation
between hstore, ltree and core. They all use the same table, so might as
well.

src/backend/utils/adt/tsgistidx.c

@@ -201,9 +201,9 @@ gtsvector_compress(PG_FUNCTION_ARGS)
 		{
 			pg_crc32	c;
 
-			INIT_CRC32(c);
-			COMP_CRC32(c, words + ptr->pos, ptr->len);
-			FIN_CRC32(c);
+			INIT_LEGACY_CRC32(c);
+			COMP_LEGACY_CRC32(c, words + ptr->pos, ptr->len);
+			FIN_LEGACY_CRC32(c);
 
 			*arr = *(int32 *) &c;
 			arr++;