Race conditions in `TransactionInitiator`

[Sword-Smith]

The taking of locks in this struct is prone to race conditions. Example.

        let selected_inputs = self
            .select_inputs(input_selection_policy, spend_amount, timestamp)
            .await?;
        trace!("selected {} inputs for transaction.", selected_inputs.len());

        let unlocked_inputs = self
            .global_state_lock
            .lock_guard()
            .await
            .unlock_inputs(selected_inputs)
            .await;

If a new transaction enters into the mempool between there two expressions, it can cause a panic in unlock_inputs. There are more problems in this method. The consolidate method on the same struct has the same problem.