open a new dataflash log, reset state

def start_new_log(self):
        '''open a new dataflash log, reset state'''
        filename = self.new_log_filepath()

        self.block_cnt = 0
        self.logfile = open(filename, 'w+b')
        print("DFLogger: logging started (%s)" % (filename))
        self.prev_cnt = 0
        self.download = 0
        self.prev_download = 0
        self.last_idle_status_printed_time = time.time()
        self.last_status_time = time.time()
        self.missing_blocks = {}
        self.acking_blocks = {}
        self.blocks_to_ack_and_nack = []
        self.missing_found = 0
        self.abandoned = 0