Bluesky: switch from firehose to jetstream

[snarfed]

Not really a pain point for us right now, our firehose consumer is stable and only 20-30% of a core, even with the firehose peaking above 1500khz right now, but this would be a good courtesy to the Bluesky team. Jetstream consumers cost them significantly less than firehose consumers, in bandwidth and CPU, and we don't currently verify signatures or use the MST blocks, so we don't really need the firehose itself.

A big chunk of this is done in a stash already.

[snarfed]

Need to prioritize this now due to bluesky-social/atproto#3036