Add RPC caching proxy between etherchain and geth

[axic]

This is especially important as while implementing accountRange the requirement from the geth team is to not support preimage resolution, but do it afterwards using the debug_preimage RPC call. This will result in a lot of RPC calls, most of which can be cached.

[jwasinger]

Unfortunately, the repo where this would be implemented is private so we would need to make the testnet devops configs publicly available somehow in order for an outside contributor to take this on.