Function submit_keep_subscribe_desired_set 

pub async fn submit_keep_subscribe_desired_set(
    backend: &BackendConn,
    securities: &[SecurityWithOpts],
) -> Result<i32, QotSubError>

Periodically replay the current CMD6211 desired set with timer_sub=1.

Ref: C++ MktQotSub.cpp:543-580 Timer_KeepSubStatus() sends the cached m_pbLastSendReq; MktQotSub.cpp:532-535 sets timer_sub=1 for that replay.