Amelioration de la gestion des connexions DB
connexions instanciées au lancement du serveur
	modified:   BackgroundTransactionHandler.java
	modified:   database/Database.java
	modified:   database/DatabaseHelper.java
	modified:   database/DatabaseManager.java
	modified:   database/mongo/MongoDatabase.java
	modified:   database/sql/SQLDatabase.java
	modified:   model/LoginManager.java
	modified:   servlets/TransactionServlet.java
	modified:   servlets/UserServlet.java
	modified:   utils/TransactionsUtils.java
	modified:   utils/UserUtils.java