Implement gorums call types as methods on Node and Configuration

[meling]

We can define func (n *Node) GRPCCall( ... ) in package gorums instead of making the Node object part of the CallData type. Similarly, other call types defined in gorums can now be defined as func (c *Configuration) QuorumCall( ... ), instead of making Nodes []*gorums.Node be part of the QuorumCallData type. We should consider if this can further simplify things.