Add option to read diskcache using remote socket dump

The diskcache tool supports dumping the diskcache over a socket connection, rather than reading from a file. We should attempt to support that in the gwdatafind server as a way of enabling better micro-service-style separation of the diskcache service from the gwdatafind service.

However, I only know how to do that using the diskcache dump command-line, and not anything in Python.