Add ETag/conditional request support to the definitions endpoint #1638

Job	Run time
Build old RabbitMQ	4m 19s
Build new RabbitMQ	4m 7s
Test durable_queue + 1 node(s)	1m 16s
Test topic_bindings_import + 1 node(s)	1m 3s
Test transient_queue + 3 node(s)	1m 17s
Test 10k_queues_import + 1 node(s)	1m 15s
Test durable_queue + 3 node(s)	1m 22s
Test topic_bindings_import + 3 node(s)	1m 51s
Test 10k_queues_import + 3 node(s)	2m 30s
	19m 0s

Provide feedback