Report streaming progress

[expede]

Works! Cleaning up...

Closes #312

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One small detail is that IPFS doesn't report the percentage, but "progress" (which is either bytes or blocks). We can estimate the percentage off of this, but given that IPFS doesn't give valid DAG sizes out of the box, it's very much a shot in the dark. As such, for now the backend will report the max progress across the cluster, and the SDK can chose what to do with that.

One strategy is to take the reported subgraph size from IPFS, and naively treat the progress as a portion of that. At worst the progress bar will jump from some low number to 100%, which is better than hanging at 99%