[services] Tunnelbroker - AMQP message payload string conversion to use size and remove double getPayload() call

Summary:
This is a small improvement and possibly a bug fix. We should use a message body size when constructing a string from an AMQP message payload instead of using body-only.

As a small follow-up presented in this diff: fix message->getPayload() double call and use the local instance instead. It's a super small change that's why merged it into a single one but a small diff to save a reviewer's time.

Linear task: ENG-1495

Test Plan: Successfully built the server using yarn run-tunnelbroker-service-in-sandbox command. Run tests using yarn run-unit-tests

Reviewers: karol, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4702