Collaboration: Add (room, type, date_gmt) index to the collaboration table.

Add a composite KEY room_type_date (room, type, date_gmt) so the
awareness read path in get_awareness_state() can seek directly to a
room's live awareness rows instead of relying on the KEY room (room,
id) prefix and post-filtering by type and date_gmt.

In the single-table design where awareness and update rows share
$wpdb->collaboration and are distinguished only by the type column,
the previous indexes left no covering path for the hot-path query
WHERE room = %s AND type = 'awareness' AND date_gmt >= %s, which runs
on every cache-miss awareness poll (~0.5-1s per active editor).
EXPLAIN on a populated table (11k rows) confirms MySQL switches from
KEY room (rows=55, Using where) to KEY room_type_date (rows=3, Using
index condition), with the full WHERE clause pushed down into the
index lookup.

The set_awareness_state() existence check (WHERE room = %s AND type
= 'awareness' AND client_id = %s) also picks up the new index as a
side benefit: its plan switches from an index_merge intersection of
type_client_id and room to a single-index seek on room_type_date.
The old intersection plan wasn't pathological, but a single composite
seek is simpler and more predictable.

Bump $wp_db_version to 61842 so dbDelta picks up the new index on
upgrade.

Author: josephfusco

src/wp-admin/includes/schema.php

@@ -198,6 +198,7 @@ function wp_get_db_schema( $scope = 'all', $blog_id = null ) {
 	PRIMARY KEY  (id),
 	KEY type_client_id (type,client_id),
 	KEY room (room,id),
+	KEY room_type_date (room,type,date_gmt),
 	KEY date_gmt (date_gmt)
 ) $charset_collate;\n";
 

src/wp-includes/version.php

@@ -23,7 +23,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 61841;
+$wp_db_version = 61842;
 
 /**
  * Holds the TinyMCE version.