Security Contexts must be used, pervasively

All security credentials used to access resources (that aren't isolated from all other workflow instances) must be derived from the credentials used to create the workflow instance.

Note that this constrains what types of security may be used: delegated credentials for service access is critical. There are multiple solutions for this though, so would be good to be flexible. Individual deployments of Taverna Server may constrain what types of security credentials are supported.