Optimization of notification_apis:

• introduced reduced notification view, providing raw account and post IDs to eliminate joins to big tables before actual filtering
• updated notification API functions to use faster view.

hive/db/sql_scripts/hive_posts_base_view.sql

+DROP VIEW if exists public.hive_posts_base_view cascade;
+
+CREATE OR REPLACE VIEW public.hive_posts_base_view
+ AS
+ SELECT 
+    hp.block_num,
+    hp.id,
+    hp.author_id,
+    hp.permlink_id,
+    hp.payout,
+    hp.pending_payout,
+    COALESCE(( SELECT sum(
+                CASE v.rshares >= 0
+                    WHEN true THEN v.rshares
+                    ELSE - v.rshares
+                END) AS sum
+           FROM hive_votes v
+          WHERE v.post_id = hp.id AND NOT v.rshares = 0
+          GROUP BY v.post_id), 0::numeric) AS abs_rshares
+   FROM hive_posts hp
+;