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/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-alpha-38924';
+$wp_version = '4.7-alpha-38925';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.