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/rpc_logging.go

@@ -0,0 +1,34 @@
+package rpc
+
+import (
+	"context"
+
+	"github.com/zrepl/zrepl/logger"
+)
+
+type Logger = logger.Logger
+
+type contextKey int
+
+const (
+	contextKeyLoggers contextKey = iota
+	contextKeyGeneralLogger
+	contextKeyControlLogger
+	contextKeyDataLogger
+)
+
+/// All fields must be non-nil
+type Loggers struct {
+	General Logger
+	Control Logger
+	Data    Logger
+}
+
+func WithLoggers(ctx context.Context, loggers Loggers) context.Context {
+	ctx = context.WithValue(ctx, contextKeyLoggers, loggers)
+	return ctx
+}
+
+func GetLoggersOrPanic(ctx context.Context) Loggers {
+	return ctx.Value(contextKeyLoggers).(Loggers)
+}