When a build completes, we store the original, unsigned rpm headers
to disk. This results in the postRPMSign callback being called, and
a rpm.sign message being sent with an empty sigkey. The intended
sematics of the rpm.sign message is to notify a service when an rpm
has been signed. Therefore, we should not be sending a message when
the sigkey is empty.
This plugin sends messages to a broker about events in the hub
using the proton library. This library supports the AMQP 1.0
protocol and is compatible with a wide variety of message
brokers. It queues all messages until the postCommit callback,
avoiding race conditions between message reception and database
transaction commit.
