PERF: Create a partial regular post_search_data index on large sites.

With the addition of `PostSearchData#private_message`, a partial
index consisting of only search data from regular posts can be created.
The partial index helps to speed up searches on large sites since PG
will not have to do an index scan on the entire search data index which
has shown to be a bottle neck.

lib/search.rb

@@ -1090,7 +1090,18 @@ class Search
       type_filter: opts[:type_filter]
     }
 
-    min_id = Search.min_post_id
+    min_id =
+      if SiteSetting.search_recent_regular_posts_offset_post_id > 0
+        if %w{all_topics private_message}.include?(opts[:type_filter])
+          0
+        else
+          SiteSetting.search_recent_regular_posts_offset_post_id
+        end
+      else
+        # This is kept around for backwards compatibility.
+        # TODO: Drop this code path after Discourse 2.7 has been released.
+        Search.min_post_id
+      end
 
     if @order == :likes
       # likes are a pain to aggregate so skip