Add message queue processor class

This handles making updates to the register based on messages received
from the queue.
Hook rabbitmq_consumer up to the processor
Set up database cleaner.

This is to allow us to use non-transactional cleaning for the message
queue integrations tests.  The message queue consumer runs in a separate
process, and is therefore outside the tests' transaction, so this is
necessary.
Make the message consumer helper wait for startup.

By waiting for a message to be written to stdout, this blocks the test
process until Rails has booted in the consumer process.  This in turn
means that the tests don't sometimes timeout because it takes too long
for the first message to be processed.
Enable the message queue integration tests.