Add gin_clean_pending_list function to clean up GIN pending list

This function cleans up the pending list of the GIN index by
moving entries in it to the main GIN data structure in bulk.
It returns the number of pages cleaned up from the pending list.

This function is useful, for example, when the pending list
needs to be cleaned up *quickly* to improve the performance of
the search using GIN index. VACUUM can do the same thing, too,
but it may take days to run on a large table.

Jeff Janes,
reviewed by Julien Rouhaud, Jaime Casanova, Alvaro Herrera and me.

Discussion: CAMkU=1x8zFkpfnozXyt40zmR3Ub_kHu58LtRmwHUKRgQss7=iQ@mail.gmail.com

doc/src/sgml/ref/create_index.sgml

@@ -362,8 +362,8 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
       Turning <literal>fastupdate</> off via <command>ALTER INDEX</> prevents
       future insertions from going into the list of pending index entries,
       but does not in itself flush previous entries.  You might want to
-      <command>VACUUM</> the table afterward to ensure the pending list is
-      emptied.
+      <command>VACUUM</> the table or call <function>gin_clean_pending_list</>
+      function afterward to ensure the pending list is emptied.
      </para>
     </note>
     </listitem>