deduplicate user metadata in the JOIN burst

[slingamn]

Right now Ergo does eager metadata sync on JOIN, including the join burst for reattaching clients. This is fine for channel metadata, but risks blowup for user metadata (e.g. the same user is in 5 of your channels; syncChannelMetadata will send you 5 copies of your subscribed keys for them, one in each sync batch).

Ideas:

1. We could deduplicate through the join burst (this adds implementation complexity)
2. For a reattaching client, we could suppress user metadata in the join burst, then send a separate metadata batch (with a * parameter for the target?) containing all deduplicated metadata for "friends".