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)

zfs/zfs_pipe_linux.go

@@ -0,0 +1,21 @@
+package zfs
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func trySetPipeCapacity(p *os.File, capacity int) {
+	res, err := unix.FcntlInt(p.Fd(), unix.F_SETPIPE_SZ, capacity)
+	if err != nil {
+		err = fmt.Errorf("cannot set pipe capacity to %v", capacity)
+	} else if res == -1 {
+		err = errors.New("cannot set pipe capacity: fcntl returned -1")
+	}
+	if debugEnabled && err != nil {
+		debug("trySetPipeCapacity error: %s\n", err)
+	}
+}