Reduce the usages of LOG(FATAL) or other self-descruction measures as many as possible.

[xtcyclist]

RT.

To improve the stability and maximize the running hours of services as much as possible.

Reasons to self-destruct:

1. Data may be corrupted or lost, if the service continues to run.
2. (Add more reasons here, if any)

[xtcyclist]

  StatusOr<IndexID> toTagIndexID(GraphSpaceID space, std::string tagName) override {
    UNUSED(space);
    UNUSED(tagName);
    LOG(FATAL) << "Unimplemented";
  }

  StatusOr<IndexID> toEdgeIndexID(GraphSpaceID space, std::string edgeName) override {
    UNUSED(space);
    UNUSED(edgeName);
    LOG(FATAL) << "Unimplemented";
  }

  Status checkTagIndexed(GraphSpaceID space, IndexID index) override {
    UNUSED(space);
    UNUSED(index);
    LOG(FATAL) << "Unimplemented";
  }

  Status checkEdgeIndexed(GraphSpaceID space, IndexID index) override {
    UNUSED(space);
    UNUSED(index);
    LOG(FATAL) << "Unimplemented";
  }

Cases like such shall be fixed. We should report errors on unimplemented features, not crashing the service.