Add resource locking commands

This adds `lock`/`unlock` commands for the following resources:

* servers
* images
* load balancers
* SQL instances
* SQL snapshots

When a resource has been locked, it may not be deleted without first unlocking it.

This can prevent accidental deletion of critical servers due to a mistyped command.

You can view the status of a resource's lock by using the `show` command (e.g. `locked: false` can be deleted)