Currently container instances are hard-coded to join a single network,
the network associated with the session.
This change allows the caller of CreateInstance to specify which
additional networks should be joined. This is useful, for example, when
a container instance requires access to additional "backend" services
that may be running.
There are security implications associated with this change, hence the
additional networks are only joined when a new -unsafe flag is
specified. It is hoped the name is a sufficient indicator that thought
needs to go into using it.
Remove global `sessions` map and use configured storage.
Add a `types` package so both `pwd` and `storage` can access without
circular dependencies.
Now the session is prepared when requested and not on load.
