zmtp: implement REQ/REP (#72)

* zmtp: improve perfs of Connection.(read|send){,Multipart}

This CL uses io.ReadFull to make sure all requested bytes are read from
an io.Reader.
It's also using binary.ByteOrder.Uint64 and binary.ByteOrder.PutUint64
directly instead of going the round about way through (slow) reflection.

Fixes #67.
Fixes #61.

* zmtp: reduce number of allocs in Connection.SendCommand

* zmtp: reduce number of allocs in Connection.writeMetadata

* zmtp: removed reflection from de/serializing greetings

* zmtp: remove slow reflection in Connection.recvMetadata

* zmtp: implement REQ/REP

Updates #65.

zmtp/socket.go

@@ -24,6 +24,10 @@ func NewSocket(socketType SocketType) (Socket, error) {
 		return dealerSocket{}, nil
 	case RouterSocketType:
 		return routerSocket{}, nil
+	case ReqSocketType:
+		return reqSocket{}, nil
+	case RepSocketType:
+		return repSocket{}, nil
 	default:
 		return nil, errors.New("Invalid socket type")
 	}
@@ -148,3 +152,41 @@ func (routerSocket) IsCommandTypeValid(name string) bool {
 	// FIXME(sbinet)
 	return false
 }
+
+type reqSocket struct{}
+
+func (reqSocket) Type() SocketType {
+	return ReqSocketType
+}
+
+func (reqSocket) IsSocketTypeCompatible(socketType SocketType) bool {
+	switch socketType {
+	case RepSocketType, RouterSocketType:
+		return true
+	}
+	return false
+}
+
+func (reqSocket) IsCommandTypeValid(name string) bool {
+	// FIXME
+	return false
+}
+
+type repSocket struct{}
+
+func (repSocket) Type() SocketType {
+	return RepSocketType
+}
+
+func (repSocket) IsSocketTypeCompatible(socketType SocketType) bool {
+	switch socketType {
+	case ReqSocketType, DealerSocketType:
+		return true
+	}
+	return false
+}
+
+func (repSocket) IsCommandTypeValid(name string) bool {
+	// FIXME
+	return false
+}