posts: fix search queries not being logged to NewRelic in some cases (#4900)

Fix the /posts index controller not logging the normalized search query
to NewRelic when the search failed, either because of a tag limit error,
a search timeout, or a RSS feed rate limit error.

Also don't log the number of search results when it's an API request or
failed search. This is to avoid doing a potentially slow full post count
when it's not otherwise needed.

app/logical/post_query_builder.rb

@@ -968,7 +968,7 @@ class PostQueryBuilder
 
   concerning :CountMethods do
     def post_count
-      fast_count
+      @post_count ||= fast_count
     end
 
     # Return an estimate of the number of posts returned by the search.  By