Macro retry_transaction

macro_rules! retry_transaction {
    ($transaction:expr) => { ... };
    (
        $transaction:expr,
        $max_retries:expr // should be an Option<int type>, None for unlimited
        $(,)?

    ) => { ... };
}

Repeatedly attempt an Optimistic Transaction until it succeeds. Since many callsites (e.g. the consensus handler) cannot proceed in the case of failed writes, this will loop forever until the transaction succeeds.