Multisite: Introduce metadata for sites.

A new global multisite table `wp_blogmeta` is added to the database schema, and a set of `*_site_meta()` API functions are introduced.

The implementation fails gracefully when the new table is not yet available, which may happen especially shortly after the core update, before the network has been upgraded to the new database schema. The presence of the table is detected once and stored as a global setting on the main network.

Core does not yet use site metadata, but there are several use-cases to be implemented or explored in the near future, and it allows plugins to extend sites with arbitrary data, which will come in particularly handy with the upcoming REST API endpoint for sites.

Props spacedmonkey, johnjamesjacoby, jeremyfelt, flixos90.
Fixes #37923.


git-svn-id: https://develop.svn.wordpress.org/trunk@42836 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/schema.php

@@ -267,6 +267,15 @@ CREATE TABLE $wpdb->blog_versions (
   PRIMARY KEY  (blog_id),
   KEY db_version (db_version)
 ) $charset_collate;
+CREATE TABLE $wpdb->blogmeta (
+  meta_id bigint(20) unsigned NOT NULL auto_increment,
+  blog_id bigint(20) NOT NULL default '0',
+  meta_key varchar(255) default NULL,
+  meta_value longtext,
+  PRIMARY KEY  (meta_id),
+  KEY meta_key (meta_key($max_index_length)),
+  KEY blog_id (blog_id)
+) $charset_collate;
 CREATE TABLE $wpdb->registration_log (
   ID bigint(20) NOT NULL auto_increment,
   email varchar(255) NOT NULL default '',