Provide a 'status' endpoint / provideway to efficiently understand when hash is indexed

[willscott]

we should consider providing a way to either:

• long poll / pub-sub to get a ping-back when a hash becomes available
• provide a non-cached endpoint for 'is this cid found'

first is probably 'better', but more involved in that it also needs to hit the ingest pipeline in a way that would take more eng

for the second, we'd keep a negative cache of recent asks in index-star so that while we wouldn't have a 5 min timeout at the http layer, we could have a e.g. 10second cache in indexstar to limit potential to ddos backends.