Enable getting the latest diskcache over a socket

This MR enables updating the diskcache over a network socket, by calling out to diskcache dump, rather than relying on reading a local file.

This is sort of hacky in that it overloads handling of the framecachefile configuration option to understand a list that gives the (host, port) pair. That should probably be hardened, or just separated out into its own option, but that can likely be handled (later) as part of Refactor configuration to use multiple sections (#33).

Implements Add option to read diskcache using remote socke... (#34 - closed).