Add GUC and storage parameter to set the maximum size of GIN pending list.

Previously the maximum size of GIN pending list was controlled only by
work_mem. But the reasonable value of work_mem and the reasonable size
of the list are basically not the same, so it was not appropriate to
control both of them by only one GUC, i.e., work_mem. This commit
separates new GUC, pending_list_cleanup_size, from work_mem to allow
users to control only the size of the list.

Also this commit adds pending_list_cleanup_size as new storage parameter
to allow users to specify the size of the list per index. This is useful,
for example, when users want to increase the size of the list only for
the GIN index which can be updated heavily, and decrease it otherwise.

Reviewed by Etsuro Fujita.

doc/src/sgml/ref/cluster.sgml

@@ -46,7 +46,7 @@ CLUSTER [VERBOSE]
    not clustered.  That is, no attempt is made to store new or
    updated rows according to their index order.  (If one wishes, one can
    periodically recluster by issuing the command again.  Also, setting
-   the table's <literal>FILLFACTOR</literal> storage parameter to less than
+   the table's <literal>fillfactor</literal> storage parameter to less than
    100% can aid in preserving cluster ordering during updates, since updated
    rows are kept on the same page if enough space is available there.)
   </para>