1）your connection user must can see the DB named “transaction”
  2）the user with credential must can [readWrite] DB-“transaction”
  4）if your collection already has an uniqueindex.you must:
     fisrt drop it,
     then recreate it but add a field named "unique_" to the index.
     i.e.: consider Collection A has  uniqueindex that name="test", field="aaaa".
           this index should be update to name="test", field="aaaa, unique_".
     !!!!OR!!!!YOU CAN CREATE INDEX VIA THIS DRIVER!!!IT WILL AUTOMATIC CREATE THE "unique_" FIELD" 
     
     
  CAUSTIONS:
  1）do not support DBRef with _id，because on update will create new temp-obj with new _id;

   SidistranMongoClient mongo = new SidistranMongoClient(serverAddress, credentials);

   MongoTanscationManager tanscationManager = new MongoTanscationManager(mongo);//this is singleton

   tanscationManager.begin();//begin a transaction

   do_with(mongo);

   tanscationManager.commit();//commit it

   tanscationManager.close();//you must close the MongoTanscationManager if won't use anymore
   mongo.close();