Making Adyen Less Insane and More Workable

- Configuration Bug - where we did not check if a file existed before
  we attempted to stat it. Also we now have better reporting when
  we provide a config file name that doesn't actually exist!

- Differentiation between an 'inflight' message and a 'pending' message
-- Inflight means we are actively processing the message RIGHT NOW
-- Pending means we will eventually get to it (e.g. it's at the processor)

- We now have only one message that comes from DonationInterface because
  I didn't want to have to figure out how to get the appropriate meta
  data into the message...

- Messages when processed now get stripped from both pending and limbo

Change-Id: If005364d62fc56465814bdeb7b3e6dac0e330b9d