Better management of Rserve connection in the WebWork macro

- rserve_start always creates a new session

- rserve_finish undefs the variable holding the Rserve session so a
  subsequent rserve_eval knows to create a new one if the user
  ommitted a call to rserve_start

- rserve_query explicitly closes the connection when it's done for an
  immediate cleanup
Fixup a bad call (f 48b6983d)