message: DumpEdges support specify some vertexId
diff: #include <rocksdb/db.h>
  DEFINE_string(path, "", "rocksdb instance path");
  +DEFINE_int64(vertex_id, 0, "Specify the vertex id");
  +DEFINE_int64(parts_num, 100, "Specify the parts number");
  namespace nebula {
  @@ -24,7 +26,13 @@ public:
  if (!iter) {
  LOG(FATAL) << "null iterator!";
  }
  + if (FLAGS_vertex_id != 0) {
  + auto partId = FLAGS_vertex_id % FLAGS_parts_num + 1;
  + auto prefix = NebulaKeyUtils::edgePrefix(partId, FLAGS_vertex_id);
  + iter->Seek(prefix);
  + } else {
  iter->SeekToFirst();
  + }
  size_t count = 0;
  while (iter->Valid()) {
  auto key = folly::StringPiece(iter->key().data(), iter->key().size());
  @@ -51,6 +59,9 @@ public:
  int main(int argc, char *argv[]) {
  folly::init(&argc, &argv, true);
  google::SetStderrLogging(google::INFO);
  + LOG(INFO) << "path:" << FLAGS_path
  + << ", vertex_id:" << FLAGS_vertex_id
  + << ", parts_num:" << FLAGS_parts_num;
  if (FLAGS_path.empty()) {
  LOG(INFO) << "Specify the path!";
  return -1;
  