Add categories to the serialized search results together with the topics
when lazy load categories is enabled. This is necessary in order for the
results to be rendered correctly and display the category information.
The global setting disable_search_queue_threshold
(DISCOURSE_DISABLE_SEARCH_QUEUE_THRESHOLD) which default to 1 second was
added.
This protection ensures that when the application is unable to keep up with
requests it will simply turn off search till it is not backed up.
To disable this protection set this to 0.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
