add bidirectional streaming support for external snapshot streaming (#266)

Author: shivaji-kharse

protos/api.v2.proto

@@ -31,7 +31,7 @@ service Dgraph {
   rpc ListNamespaces(ListNamespacesRequest) returns (ListNamespacesResponse) {}
 
   rpc UpdateExtSnapshotStreamingState(UpdateExtSnapshotStreamingStateRequest) returns (UpdateExtSnapshotStreamingStateResponse) {}
-  rpc StreamExtSnapshot(stream StreamExtSnapshotRequest) returns (StreamExtSnapshotResponse) {}
+  rpc StreamExtSnapshot(stream StreamExtSnapshotRequest) returns (stream StreamExtSnapshotResponse) {}
 }
 
 message PingRequest {}
@@ -178,7 +178,9 @@ message StreamExtSnapshotRequest {
   StreamPacket pkt = 3;
 }
 
-message StreamExtSnapshotResponse {}
+message StreamExtSnapshotResponse {
+  bool finish = 1;
+}
 
 message StreamPacket {
   bytes data = 1;