Include unsubscribes in the forum subscription cache to improve performance

When looking to some DB performance problems @ MDL-65069, it was detected that this query:

SELECT id, discussion, preference FROM mdl_forum_discussion_subs WHERE userid = $1 AND forum = $2 array (

  0 => '2',

  1 => 107,

)

Is executed 65 times in a forum/view page plenty of discussions.

Tracing down the problem... it seems that mod_foum\subscriptions::fill_discussion_subscription_cache() does not cache query misses... and that causes the same query to be executed as many times as discussions are in a page.

This should be fixed in all supported branches because they are lots of queries for nothing.