Use redis pub/sub instead of polling to monitor pending transfers

[jhford]

Currently in the /redirect/ endpoint, if we encounter a file in the pending state, we poll every second until we see that pending status change to either 'error' or 'present'. We should probably have a failsafe of polling every 10 seconds to make sure that pub/sub being flakey doesn't cause too much trouble. In this case, we should log something to make it clear that we have experienced this condition.

[jhford]

The best place to start here is in src/api-v1.js in the /redirect/ method. The functions which are also involved are in src/storage-backend.js in the StorageBackend class. The functions are getBackendUrl, storeAddress and readAddressFromCache