This change implements a mechanism to retransmit orders if some of the
order's selected regions do not confirm transmission in due time. It
adds a worker to repeatedly check the orders and determine if they need
retransmission. Such orders will be added to a new table named
tx_retries. The tx_start function now first checks if there are regular
new paid orders to transmit. If not, it will check the retransmission
table and retransmit an order from there if one is available.
This patch also introduces a new order state called "retranmission". The
order enters this state while waiting for retransmission.
Users can provide a list of regions to POST /order requests. This list
indicates on which regions the order should be transmitted. It also
determines the expected transmission and reception confirmations.
