pipeline event stream playback

[whyrusleeping]

The database event persisters playback loop needs a bit of optimization.
Relevant code here:
https://github.com/bluesky-social/indigo/blob/main/events/dbpersist.go#L128

The easy first pass should be to just pipeline the database loading and the event hydration. Also some amount of parallelism and/or caching in the event hydration is likely warranted too.