Added getter for WebhookManager to be used in tests
Replaced String.format with SLF4J inline string substitution. Use lower case for logger since it is not a constant
Fixed WebhookManager to be a LeaderLatchListener so that queues are being started only when the instance is a leader and are stopped when seizes being a leader. In this way only the leader consumes web hook queues (to guarantee ordered processing) but all instances can handle webhook requests
Added dependencies and base code for unit testing. Added initial WebhookResource and WebhookManager tests