Changes to make ACS better for node apps

for node apps that use ACS w/o request/response callbacks, should be able to continue to use sessionID from singleton ACS instance.  this works by caching the sessionID and using it if not provided in the cookie of the request or not provided in the data object.

don't require a callback function when invoking ACS.  If i don't care about return result, i shouldn't have to provide a callback.