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.
This change removes the infinite transmission loop with sleep periods
between each transmission. Now, a new transmission can start by a call
to "tx_start" on the following two conditions:
1) As soon as the application starts (if there is a previously paid
order waiting already).
2) As soon as the current transmission ends. The server will immediately
look for the next order ready for transmission and start it if
available.
Meanwhile, the condition for ending the current transmission (i.e., for
calling "tx_end(order)") is when all Tx confirmations are received from
all regions.
