Moving walls on Oasis in a single transaction (via `TxManager`)

It might be possible to kill and recreate orders in a single Ethereum transaction (via a TxManager for example) for the oasis-market-maker-keeper. Currently there is a significant delay between when the order gets killed and when its replacement gets created, and during that time this specific band is not covered by the keeper in the orderbook.

This might not be trivial to implement as the killed order may be partially or completely taken by some other actor before the "combo" transaction gets mined, which means we may not be able to know what the pay_amount of the new order should be. The advantage of doing it in individual transactions (as it is now) is the fact that we can always be sure of that amount.