Following ADDR 008 the node will connect to an external
process to handle signing requests. Operation of the external process is
left to the user.
* introduce alias for PrivValidator interface on socket client
* integrate socket client in node
* structure tests
* remove unnecessary flag
As calls to the private validator can involve side-effects like network
communication it is desirable for all methods returning an error to not
break the control flow of the caller.
* adjust PrivValidator interface