Feeds: Always return a valid timestamp for the Last-Modified header of comment or post feeds.

Fixes bug where an invalid Last-Modified value would be returned in feed requests for sites that had 0 items to return. Comment or post feeds will now return the current timestamp as the Last-Modified header value.  Example: a request for the comments feed for a site without any comments.

Replaced use of the local static variable `$cache_lastcommentmodified` to store the modified date in `get_lastcommentmodified()` with the Object Cache API.  The `get_lastcommentmodified()` function returns early if there is a cached value and returns `false` if there where no comments found. Introduced `_clear_modified_cache_on_transition_comment_status()` to flush the `lastcommentmodified` cache key when a comment enters or leaves approval status. In `get_lastpostmodified()` return early if there is a cached value and return `false` if there are no posts found.

Props swissspidy, rachelbaker, dllh, leobaiano.
Fixes #38027.
Built from https://develop.svn.wordpress.org/trunk@38925


git-svn-id: http://core.svn.wordpress.org/trunk@38868 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/feed-rss2.php

@@ -42,7 +42,10 @@ do_action( 'rss_tag_pre', 'rss2' );
 	<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
 	<link><?php bloginfo_rss('url') ?></link>
 	<description><?php bloginfo_rss("description") ?></description>
-	<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
+	<lastBuildDate><?php
+		$date = get_lastpostmodified( 'GMT' );
+		echo $date ? mysql2date( 'D, d M Y H:i:s +0000', $date ) : date( 'D, d M Y H:i:s +0000' );
+	?></lastBuildDate>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 	<sy:updatePeriod><?php
 		$duration = 'hourly';