Fix memory leak: delete pending reply callback after reply
The exchange has a memory leak that's only noticeable in high volume traffic. We append a callback function to pendingReplies but never remove it once it's called.
Here are some screenshots of our worker memory before and after this fix:

After:

Awesome, @tehkaiyu - thanks for the contribution! It looks like the build is failing when testing this change; can you investigate? Will gladly merge the PR once we verify it's a non-breaking change.
Yep I'll take a look this weekend
Hmm, tests seem to pass locally, any chance this is an issue with the travis setup?

Travis logs seem to indicate that it can't connect to the amqp port: https://travis-ci.org/hunterloftis/jackrabbit/jobs/548540256
Uncaught Error: connect ECONNREFUSED 127.0.0.1:5672