Convert rpc module to RpcRunner

This big patch changes the call model used in internode-rpc from
standalong function calls in the rpc module to via a RpcRunner class,
that holds all the methods. This can be used in the future to enable
smarter processing in the RPC layer itself (some quick examples are not
setting the DiskID from cmdlib code, but only once in each rpc call,
etc.).

There are a few RPC calls that are made outside of the LU code, and
these calls are left as staticmethods, so they can be used without a
class instance (which requires a ConfigWriter instance).

Reviewed-by: imsnah