✨ Add adjustable prefetch limit to listeners and endpoints

This uses the 'global' flag to achieve per-channel behaviour in RabbitMQ v3.3.0+.
We already use 3.5.0+ for priority queues, so we're safe here.

lib/Endpoint.js

@@ -118,7 +118,7 @@ class Endpoint {
     }
   }
 
-  async _setup ({ queue, event }) {
+  async _setup ({ queue, event, prefetch = 48 }) {
     try {
       const worker = await this._remit._workers.acquire()
 
@@ -142,7 +142,10 @@ class Endpoint {
       this._consumer.on('close', () => {
         throwAsException(new Error('Consumer died - this is most likely due to the RabbitMQ connection dying'))
       })
-      this._consumer.prefetch(48)
+
+      if (prefetch > 0) {
+        this._consumer.prefetch(prefetch, true)
+      }
 
       await this._consumer.bindQueue(
         queue,

lib/Listener.js

@@ -94,7 +94,7 @@ class Listener {
     this._consumer.ack(message)
   }
 
-  async _setup ({ queue, event }) {
+  async _setup ({ queue, event, prefetch = 48 }) {
     try {
       const worker = await this._remit._workers.acquire()
 
@@ -118,7 +118,10 @@ class Listener {
       this._consumer.on('close', () => {
         throwAsException(new Error('Consumer died - this is most likely due to the RabbitMQ connection dying'))
       })
-      this._consumer.prefetch(48)
+
+      if (prefetch > 0) {
+        this._consumer.prefetch(prefetch, true)
+      }
 
       await this._consumer.bindQueue(
         queue,