saved searches: fix failure when search returns no results.

* Don't try to call `sadd` when a search returns no results (`sadd`
  fails in this case).
* Add a timeout when populating the search.
* Don't offload the search to read replica. The main db is fine.
* Disable synchronous population of searches. This was too slow.

app/models/application_record.rb

@@ -266,7 +266,7 @@ class ApplicationRecord < ActiveRecord::Base
         connection.execute("SET STATEMENT_TIMEOUT = #{n}") unless Rails.env == "test"
         yield
       rescue ::ActiveRecord::StatementInvalid => x
-        DanbooruLogger.log(x, expected: true)
+        DanbooruLogger.log(x, expected: false, **new_relic_params)
         return default_value
       ensure
         connection.execute("SET STATEMENT_TIMEOUT = #{CurrentUser.user.try(:statement_timeout) || 3_000}") unless Rails.env == "test"