refactor get().
refactor get().

invoke_async will now (confusingly) be invoked synchonously when wait_on_generate==True unless async_wait_on_generate==True

flag is only fetched when it is needed to lock the invoke_async call

flag will now also be automatically corrected when it has a negative value
	this is thought to occur only when the flag expires between an incr() and a decr(). such as when someone is messing around with the cache manually

artfinder/django-jcache