Modify hash_search() API to prevent future occurrences of the error

spotted by Qingqing Zhou.  The HASH_ENTER action now automatically
fails with elog(ERROR) on out-of-memory --- which incidentally lets
us eliminate duplicate error checks in quite a bunch of places.  If
you really need the old return-NULL-on-out-of-memory behavior, you
can ask for HASH_ENTER_NULL.  But there is now an Assert in that path
checking that you aren't hoping to get that behavior in a palloc-based
hash table.
Along the way, remove the old HASH_FIND_SAVE/HASH_REMOVE_SAVED actions,
which were not being used anywhere anymore, and were surely too ugly
and unsafe to want to see revived again.

contrib/dblink/dblink.c

@@ -2061,11 +2061,6 @@ createNewConnection(const char *name, remoteConn * con)
 	hentry = (remoteConnHashEnt *) hash_search(remoteConnHash, key,
 											   HASH_ENTER, &found);
 
-	if (!hentry)
-		ereport(ERROR,
-				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("out of memory")));
-
 	if (found)
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),

contrib/tablefunc/tablefunc.c

@@ -144,10 +144,6 @@ do { \
 	snprintf(key, MAX_CATNAME_LEN - 1, "%s", CATDESC->catname); \
 	hentry = (crosstab_HashEnt*) hash_search(crosstab_HashTable, \
 										 key, HASH_ENTER, &found); \
-	if (hentry == NULL) \
-		ereport(ERROR, \
-				(errcode(ERRCODE_OUT_OF_MEMORY), \
-				 errmsg("out of memory"))); \
 	if (found) \
 		ereport(ERROR, \
 				(errcode(ERRCODE_DUPLICATE_OBJECT), \