Unsubscribe From Channel

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

1. Avid Connector API sends payload
   with bal_to_sg_unsubscribe 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;