don't add session to timer/interval RC

* the session often does not have much to do with the timer call and
  is therefore confusing (e.g.: NPC movement launched in some arbitrary
  player session)
* the reference to the old session unnecessarily keeps it in memory
* somewhat related API function stub change ("failed" pathing due to a
  falsy return value constantly triggers new pathing requests)
Merge pull request #55 from yelworc/no-session-for-timers

don't add session to timer/interval RC