Detaches a ModelExposer from the current thread - enabling it to be garbage collected
if necessary (assuming ThreadLocal had the last remaining reference to the ModelExposer).
Genesis objects are the essential, persistent, system objects that must be created
right at the beginning of the life of a new system and from which all other objects
will go forth and multiply.
A servlet filter that automatically implements dependency injection by calls to attachThread
at the start of doFilter and detachThread after the filter chain/servlet has generated
the response.