Generally allow customer to repay payment. The work was based on:

 * Start date do not have to be set during building.
 * Allow to reuse existing ID and allow PayZen to inform user, that transaction
   is already registered.
 * Do not create too many ids.
 * Drop not needed logging.
 * Be as precise as invoice's currency.
 * Consider all planned or confirmed transactions and take actions based on
   transaction status.