Optimize tac_lite_node_grants with static variable

tac_lite_node_grants() checks users grants for each node on the requested page. On node summary or list pages this happens many times and is a waste of processing since the grants are not node specific. The following patch creates a static variable $grants that is keyed by user ID and node op. The data stored in the static variable is very small. Also always returns an array (even if empty) as specified in hook_node_grants().