Simplify pg_mcv_list (de)serialization

The serialization format of multivariate MCV lists included alignment in
order to allow direct access to part of the serialized data, but despite
multiple fixes (see for example commits d85e0f36 and ea4e1c0e) this
proved to be problematic.

This commit abandons alignment in the serialized format, and just copies
everything during deserialization.  We now also track amount of memory
needed after deserialization (including alignment), which allows us to
deserialize the MCV list in a single pass.

Bump catversion, as this affects contents of pg_statistic_ext_data.

Backpatch to 12, where multi-column MCV lists were introduced.

Author: Tomas Vondra
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/2201.1561521148@sss.pgh.pa.us

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201907051
+#define CATALOG_VERSION_NO	201907052
 
 #endif

src/include/statistics/extended_stats_internal.h

@@ -36,6 +36,7 @@ typedef struct DimensionInfo
 {
 	int			nvalues;		/* number of deduplicated values */
 	int			nbytes;			/* number of bytes (serialized) */
+	int			nbytes_aligned;	/* size of deserialized data with alignment */
 	int			typlen;			/* pg_type.typlen */
 	bool		typbyval;		/* pg_type.typbyval */
 } DimensionInfo;