Add delegate API in multus-daemon (#890)

This changes introduce delegate API function in multus-daemon.
This API will be consumed from other programs for hot-plug
interface into running pod. This change also cleanups server
code to split into client code and server code to easy to import
from other golang code.

pkg/server/types.go

@@ -4,8 +4,6 @@ import (
 	"net/http"
 
 	"github.com/containernetworking/cni/pkg/invoke"
-	"github.com/containernetworking/cni/pkg/skel"
-	cni100 "github.com/containernetworking/cni/pkg/types/100"
 
 	"github.com/prometheus/client_golang/prometheus"
 
@@ -13,16 +11,6 @@ import (
 	"gopkg.in/k8snetworkplumbingwg/multus-cni.v3/pkg/types"
 )
 
-type cniRequestFunc func(cmd string, k8sArgs *types.K8sArgs, cniCmdArgs *skel.CmdArgs, exec invoke.Exec, kubeClient *k8sclient.ClientInfo) ([]byte, error)
-
-// Request sent to the Server by the multus-shim
-type Request struct {
-	// CNI environment variables, like CNI_COMMAND and CNI_NETNS
-	Env map[string]string `json:"env,omitempty"`
-	// CNI configuration passed via stdin to the CNI plugin
-	Config []byte `json:"config,omitempty"`
-}
-
 // Metrics represents server's metrics.
 type Metrics struct {
 	requestCounter *prometheus.CounterVec
@@ -32,7 +20,6 @@ type Metrics struct {
 // the CNI shim requests issued when a pod is added / removed.
 type Server struct {
 	http.Server
-	requestFunc  cniRequestFunc
 	rundir       string
 	kubeclient   *k8sclient.ClientInfo
 	exec         invoke.Exec
@@ -40,12 +27,6 @@ type Server struct {
 	metrics      *Metrics
 }
 
-// Response represents the response (computed in the CNI server) for
-// ADD / DEL / CHECK for a Pod.
-type Response struct {
-	Result *cni100.Result
-}
-
 // ShimNetConf for the shim cni config file written in json
 type ShimNetConf struct {
 	types.NetConf