migrate filecoin-pin to use the StorageManager for managing storage contexts

[SgtPooki]

We should route filecoin-pin operations through the StorageManager synapse.storage instead of interacting with contexts directly.

I am not sure this is fully possible, nor that it's the desired path, but I wanted to open this issue to track.

cc @BigLep

related to #262

[rvagg]

Yes, we should aim to do this, we'll need to be doing multi-uploads, which either means messing with multiple contexts or getting StorageManager to handle it for us. We just need to get the API right for multi-context upload(); it's not quite right yet and that's one of the main hold-ups.