Changes

• Added startTX() and endTX() to inhibit gc() so that intermediate nodes would not be garbage collected unintentionally. All added tryGC(), which only perform gc() when it see fit. tryGC() is called in endTX() if transaction level becomes 0.
• Newly created node is considered dead and increments the dead nodes counter.
• Added allSat(int bdd, List<boolean[]>ret).