Provide more warning against using the cleartext state lease storage plugin.

Adding a requirement check might be a bit excessive for this change so decided against it for the time being. Long term we may want to add such a check and deprecate the cleartext plugin.

While we could technically do so now as we are in Alpha, the plugin has been around since the original 1.x alpha branches, it would be aggressive to remove it at this juncture when trying to bring the module to a stable release, especially with consideration that the Lockr module will go unsupported at the end of November when the service shuts down.

Added a new static memory plugin that can be used instead of encrypted storage for sites that do not expect to use Leases frequently (some/many? sites may never use them) to avoid the need for setting up the Encrypt module.