introduce round robin fallback connection strings

[john-behm-bertelsmann]

The general idea is that we want to have fallback rabbitmq instances in case that the main one becomes unavailable.

The reconnection loop should try to connect to the first (main) rabbitmq on every retry.
If that fails, then the other fallback urls should be tested one by one.

[jxsl13]

primary url
fallback 1
fallback 2
...
fallback n

-> []string{primary, fallback 1, primary, fallback 2, ..., primary, fallback n}
-> round robin