Subscribe To Channel

When subscribeToChannel operation is invoked on Avid Connector API it is
following next steps:

1. Avid Connector API sends payload
   with bal_to_sg_subscribe action to Secure Gateway;
   1. Avid Connector API is waiting (non-blocking/async) for incoming
      payload having action sg_to_gal_acknowledged;
   2. When payload received it
      checks metadata→runtime→errorType;
      1. If errorType doesn’t exist, notify caller about success;
      2. If errorType was provided, notify caller about corresponding
         failure;

When subscribed to channel(s), Avid Connector API
receives Channel Message

1. When Secure Gateway receives from
   RabbitMQ Channel Message,
   it sends payload with sg_to_bal_post action to Avid Connector
   API.
2. When Avid Connector API receives payload
   with sg_to_bal_post action, it must take subscriberId from
   payload and invoke corresponding handler.