Added "constants" for DatabaseOperation wrappers

For consistence sake, we should have "constants" for all relevant DatabaseOperations sub-classes, including those that are wrappers (like CloseConnectionOperation and TransactionOperation).

Of course, such "constants" cannot be variables, as they require a parameter (the operation being wrapped), but methods instead:

public static final DatabaseOperation TRANSACTION(DatabaseOperation operation) {
return new TransactionOperation(operation);
}
public static final DatabaseOperation CLOSE_CONNECTION(DatabaseOperation operation) {
return new CloseConnectionOperation(operation);
}