rpc rewrite: control RPCs using gRPC + separate RPC for data transfer

transport/ssh: update go-netssh to new version
    => supports CloseWrite and Deadlines
    => build: require Go 1.11 (netssh requires it)

transport/tcp/connect_tcp.go

@@ -0,0 +1,30 @@
+package tcp
+
+import (
+	"context"
+	"net"
+
+	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/transport"
+)
+
+type TCPConnecter struct {
+	Address string
+	dialer  net.Dialer
+}
+
+func TCPConnecterFromConfig(in *config.TCPConnect) (*TCPConnecter, error) {
+	dialer := net.Dialer{
+		Timeout: in.DialTimeout,
+	}
+
+	return &TCPConnecter{in.Address, dialer}, nil
+}
+
+func (c *TCPConnecter) Connect(dialCtx context.Context) (transport.Wire, error) {
+	conn, err := c.dialer.DialContext(dialCtx, "tcp", c.Address)
+	if err != nil {
+		return nil, err
+	}
+	return conn.(*net.TCPConn), nil
+}