+ sleep_interval: Number of seconds to sleep before each download when
+ used alone or a lower bound of a range for randomized
+ sleep before each download (minimum possible number
+ of seconds to sleep) when used along with
+ max_sleep_interval.
+ max_sleep_interval:Upper bound of a range for randomized sleep before each
+ download (maximum possible number of seconds to sleep).
+ Must only be used along with sleep_interval.
+ Actual sleep time will be a random float from range
+ [sleep_interval; max_sleep_interval].