Refactor boosts so that they are stored per-post

[julianlam]

Currently, when an Announce activity is received, a topic event is added to the topic (with the referenced pid in the topic event object)

This makes it slightly harder to determine how many Announces a post has received. You'd have to traverse all of the events of that topic in order to count them.

It would be more performant to save them separately (e.g. pid:<pid>:announces) and dynamically populate them (a la post queue) in the topic.