use monitors when requesting globally pinned chunks

[mortelli]

When swarm.netStore fails to retrieve a globally pinned chunk, it will call a hook function to send a trojan message to trigger the recovery of if (i.e., have it re-uploaded to the network) to one of the pinners.

As it is, this call results in a pss.Monitor object which the requester is not doing anything with. We could use this to track the progress of the chunk repair before retrying a fetch.

Additionally, we should look into how to deal with this when there are multiple targets to request chunk repair.