Create a dummy API Service Server

[lizhefeng]

We want a dummy API service server that implements all the functions defined in the rpc proto: https://github.com/iotexproject/iotex-election/blob/master/pb/api/api.proto. Every function should return an empty response instead.
Also add a dummyServer flag in the config: https://github.com/iotexproject/iotex-election/blob/master/server/server.go#L42. So when creating a new server, we can decide between the real server and the dummy server.

[willstudy]

WIP