[services] Tunnelbroker - Refactor handler names in Amqp connectInternal()

Summary:
This diff is a part of the stack.

Introducing refactoring to the handler variable names in AmqpManager -> connectInternal() method to reflect its purposes instead of using the shorthand.

Related linear task: ENG-1495

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/D4743