DEV: remove exec_sql and replace with mini_sql

Introduce new patterns for direct sql that are safe and fast.

MiniSql is not prone to memory bloat that can happen with direct PG usage.
It also has an extremely fast materializer and very a convenient API

- DB.exec(sql, *params) => runs sql returns row count
- DB.query(sql, *params) => runs sql returns usable objects (not a hash)
- DB.query_hash(sql, *params) => runs sql returns an array of hashes
- DB.query_single(sql, *params) => runs sql and returns a flat one dimensional array
- DB.build(sql) => returns a sql builder

See more at: https://github.com/discourse/mini_sql

app/models/incoming_link.rb

@@ -89,10 +89,10 @@ class IncomingLink < ActiveRecord::Base
 
   # Internal: Update appropriate link counts.
   def update_link_counts
-    exec_sql("UPDATE topics
+    DB.exec("UPDATE topics
               SET incoming_link_count = incoming_link_count + 1
               WHERE id = (SELECT topic_id FROM posts where id = ?)", post_id)
-    exec_sql("UPDATE posts
+    DB.exec("UPDATE posts
               SET incoming_link_count = incoming_link_count + 1
               WHERE id = ?", post_id)
   end