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)

rpc/dataconn/dataconn_shared.go

@@ -0,0 +1,70 @@
+package dataconn
+
+import (
+	"io"
+	"sync"
+	"time"
+
+	"github.com/zrepl/zrepl/rpc/dataconn/stream"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+const (
+	EndpointSend string = "/v1/send"
+	EndpointRecv string = "/v1/recv"
+)
+
+const (
+	ReqHeader uint32 = 1 + iota
+	ReqStructured
+	ResHeader
+	ResStructured
+	ZFSStream
+)
+
+// Note that changing theses constants may break interop with other clients
+// Aggressive with timing, conservative (future compatible) with buffer sizes
+const (
+	HeartbeatInterval         = 5 * time.Second
+	HeartbeatPeerTimeout      = 10 * time.Second
+	RequestHeaderMaxSize      = 1 << 15
+	RequestStructuredMaxSize  = 1 << 22
+	ResponseHeaderMaxSize     = 1 << 15
+	ResponseStructuredMaxSize = 1 << 23
+)
+
+// the following are protocol constants
+const (
+	responseHeaderHandlerOk          = "HANDLER OK\n"
+	responseHeaderHandlerErrorPrefix = "HANDLER ERROR:\n"
+)
+
+type streamCopier struct {
+	mtx                sync.Mutex
+	used               bool
+	streamConn         *stream.Conn
+	closeStreamOnClose bool
+}
+
+// WriteStreamTo implements zfs.StreamCopier
+func (s *streamCopier) WriteStreamTo(w io.Writer) zfs.StreamCopierError {
+	s.mtx.Lock()
+	defer s.mtx.Unlock()
+	if s.used {
+		panic("streamCopier used mulitple times")
+	}
+	s.used = true
+	return s.streamConn.ReadStreamInto(w, ZFSStream)
+}
+
+// Close implements zfs.StreamCopier
+func (s *streamCopier) Close() error {
+	// only record the close here, what we do actually depends on whether
+	// the streamCopier is instantiated server-side or client-side
+	s.mtx.Lock()
+	defer s.mtx.Unlock()
+	if s.closeStreamOnClose {
+		return s.streamConn.Close()
+	}
+	return nil
+}