Use Peer.ID instead of Peer.Key as peer identifier (#664)

Replace Peer.Key as internal identifier with a randomly generated Peer.ID 
in the Management service.
Every group now references peers by ID instead of a public key.
Every route now references peers by ID instead of a public key.
FileStore does store.json file migration on startup by generating Peer.ID and replacing
all Peer.Key identifier references .

management/server/file_store_test.go

@@ -244,6 +244,7 @@ func TestFileStore_SavePeerStatus(t *testing.T) {
 	// save new status of existing peer
 	account.Peers["testpeer"] = &Peer{
 		Key:      "peerkey",
+		ID:       "testpeer",
 		SetupKey: "peerkeysetupkey",
 		IP:       net.IP{127, 0, 0, 1},
 		Meta:     PeerSystemMeta{},