Bug: http request backoff unit milliseconds treated as second

[qytz]

according to https://github.com/binance/binance-connector-python/blob/master/common/src/binance_common/configuration.py#L50

the unit of backoff filed is milliseconds, but

https://github.com/binance/binance-connector-python/blob/master/common/src/binance_common/utils.py#L318

https://github.com/binance/binance-connector-python/blob/master/common/src/binance_common/utils.py#L409

the place using this filed treated it as unit second

[qytz]

#392