Merge branch 'mt-get-discussion-methods-fixes' into 'develop'

A simplification of `get_discussions*` methods

See merge request !251

(cherry picked from commit e8b65adf)

50275aad A simplification of `get_discussions*` methods
ddc9f3d9 The method `pids_by_query` is definitely removed
6b71f0db Better sort in `condenser_get_discussions_by_hot` SQL function
55cbcfd3 Better sort in `condenser_get_discussions_by_promoted` SQL method
0db6106f Added index `is_paidout` into `hive_posts` table
c8eca37b hive_posts_is_paidout_idx index shall be disabled during massive sync.