Identify if the given session ID is currently valid.
		
		Return True if valid, False if explicitly invalid, None if unknown.

def is_valid(self, context, sid):
		"""Identify if the given session ID is currently valid.
		
		Return True if valid, False if explicitly invalid, None if unknown.
		"""
		
		record = self._Document.find_one(sid, project=('expires', ))
		
		if not record:
			return
		
		return not record._expired