Benchmarks will fail if containers are stopped without removing volume directories

[jc-harrison]

FlowDBConfig checks whether FlowDB has been created for the current set of parameters using the is_created property. This checks whether the volume directory exists, but doesn't check whether the associated FlowDB container is running. If for any reason the container has been stopped without removing the volume directory, FlowDBConfig will fail when it expects to find a container which is not running.

Either is_created should also check for the container, or we should have two properties, volume_is_created and container_is_created.