Improve ilist.h's support for deletion of slist elements during iteration.

Previously one had to use slist_delete(), implying an additional scan of
the list, making this infrastructure considerably less efficient than
traditional Lists when deletion of element(s) in a long list is needed.
Modify the slist_foreach_modify() macro to support deleting the current
element in O(1) time, by keeping a "prev" pointer in addition to "cur"
and "next".  Although this makes iteration with this macro a bit slower,
no real harm is done, since in any scenario where you're not going to
delete the current list element you might as well just use slist_foreach
instead.  Improve the comments about when to use each macro.

Back-patch to 9.3 so that we'll have consistent semantics in all branches
that provide ilist.h.  Note this is an ABI break for callers of
slist_foreach_modify().

Andres Freund and Tom Lane

src/backend/postmaster/pgstat.c

@@ -3598,6 +3598,13 @@ pgstat_write_statsfiles(bool permanent, bool allDbs)
 	{
 		slist_mutable_iter iter;
 
+		/*
+		 * Strictly speaking we should do slist_delete_current() before
+		 * freeing each request struct.  We skip that and instead
+		 * re-initialize the list header at the end.  Nonetheless, we must use
+		 * slist_foreach_modify, not just slist_foreach, since we will free
+		 * the node's storage before advancing.
+		 */
 		slist_foreach_modify(iter, &last_statrequests)
 		{
 			DBWriteRequest *req;