buffer notif writes

[roadscape]

During sync, writes to hive_notifs writes should be queued and flushed.

INFO:hive.utils.stats:Service: db -- 224s total (71.1%)
INFO:hive.utils.stats:  -pct-     -ttl-     -avg-     -cnt-
INFO:hive.utils.stats:  21.0%   47018ms      1.79    26199x -- INSERT INTO hive_notifs (type_id, score, created_at, src_id, dst_id, post_id, communit ... S (:type_id, :score, :created_at, :src_id, :dst_id, :post_id, :community_id, :payload)
INFO:hive.utils.stats:  13.3%   29881ms      1.54    19344x -- SELECT id FROM hive_posts WHERE author = :a AND permlink = :p
INFO:hive.utils.stats:   9.4%   21068ms      1.37    15395x -- SELECT 1 FROM hive_notifs WHERE dst_id = :dst_id AND src_id = :src_id AND post_id = :post_id AND type_id = 17