[services] Tunnelbroker - Changes in Amqp messages testing to send and listen in different threads

Summary:
In real-world usage sending and receiving messages in the AMQP client (AMQP Manager) would be in different threads.
This diff changes sending messages from another than the main thread. Listening (message pop) is in blocking mode in the main thread without changes.

Related Linear task: ENG-1491

Test Plan:
Successfully built using yarn run-tunnelbroker-service-in-sandbox command.
Passing all AMQP unit tests in the last diff D4749 in a stack.

Reviewers: karol, tomek

Reviewed By: karol, tomek

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

Differential Revision: https://phab.comm.dev/D4746