grpc_wait_all - Waits until all sessions have ended.
  grpc_error_t grpc_wait_all()
  NoneThe grpc_wait_all() function waits for all of the executing sessions to end. When no executing sessions exist, the grpc_wait_all() returns GRPC_NOERROR.
This function is MT-unsafe.
If successful, GRPC_NO_ERROR is returned. In the case of an error, Error code is returned.
  GRPC_NOT_INITIALIZED  GRPC_SESSION_FAILED  GRPC_TIMEOUT_NP  GRPC_OTHER_ERROR_CODE