Better default value for expiration of sessions

The default value expiration duration of a session is currently hard coded in the [request model](https://github.com/ul-mds/fable-model/blob/63cf4e69fe4f0a03e476003c9c0fa7aaf2094d17/fable_model/broker.py#L17). The default value should match the value defined in the env var `MAX_SESSION_TIMEOUT`.