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.go

@@ -0,0 +1,18 @@
+// +build !linux
+
+package zfs
+
+import (
+	"os"
+	"sync"
+)
+
+var zfsPipeCapacityNotSupported sync.Once
+
+func trySetPipeCapacity(p *os.File, capacity int) {
+	if debugEnabled {
+		zfsPipeCapacityNotSupported.Do(func() {
+			debug("trySetPipeCapacity error: OS does not support setting pipe capacity")
+		})
+	}
+}