Only create tmp dir if doesn't exists
Support multiple Fallback caches

The Fallback engine configures 2 caches internally. The 2 configurations
for these caches where initally called 'primary' and 'secondary'. But
this creates a conflict if multiple Fallback caches are creates.
Now require a 'name' setting for the Fallback engine and use it to
create unique config names for the 2 internal engines.
Also make sure to call parent:init() as this does some internal work
like converting duration when it's set with '+x days' for instance.
Call parent::init()

Call parent::init() in the init() function because it does sone internal
(and useful) work such as converting the 'duration' setting to seconds
(if it's set with something like '+x days')
_clearDirectory(0 not needed.

Function does not need to be overridden. It deletes directory correctly.
The issue was due to a missunderstanding of how the 'prefix' setting was
being used.