notif aggregation

[roadscape]

Benefits of aggregation:

• filter less raw notifications
• better organization, density
• informative re-engagement

Notifications to aggregate:

• all votes, or votes on a specific post (agg = voters, posts, rshares_sum)
• new replies to a specific post (agg = post, comments, authors)
• post/curation/beneficiary payouts (agg = total sbd/steem/sp)

Unclear if the agg key should include date, and/or if it should be a 2nd level of aggregation on retrieval. (By day of year would be too many for inactive users)