Make catalog cache hash tables resizeable.

If the hash table backing a catalog cache becomes too full (fillfactor > 2),
enlarge it. A new buckets array, double the size of the old, is allocated,
and all entries in the old hash are moved to the right bucket in the new
hash.

This has two benefits. First, cache lookups don't get so expensive when
there are lots of entries in a cache, like if you access hundreds of
thousands of tables. Second, we can make the (initial) sizes of the caches
much smaller, which saves memory.

This patch dials down the initial sizes of the catcaches. The new sizes are
chosen so that a backend that only runs a few basic queries still won't need
to enlarge any of them.

src/include/utils/catcache.h

@@ -66,8 +66,8 @@ typedef struct catcache
 	long		cc_lsearches;	/* total # list-searches */
 	long		cc_lhits;		/* # of matches against existing lists */
 #endif
-	dlist_head	cc_bucket[1];	/* hash buckets --- VARIABLE LENGTH ARRAY */
-} CatCache;						/* VARIABLE LENGTH STRUCT */
+	dlist_head *cc_bucket;		/* hash buckets */
+} CatCache;
 
 
 typedef struct catctup