txn: speed up and streamline the API

Speed up txn construction and make txn object smaller:
* initialize triggers in txn on first use
* remove unnecessary members of txn
* replace rlist with slist (double linke list with single linke list)
  for statement list; implement a nicer single linked list for
  that purpose, and cover it with tests;

Streamline engine API:
* replace beginStatement with begin(),
  engines have no need to be alerted on each statement
* don't commit statement transaction inside the engine,
  do it once outside engine API
* TupleGuard -> TupleRef, implement TupleRefNil