change network retry delay strategy

[guolinke]

to fix #2348

[StrikerRUS]

Can't we simply expose connect_fail_retry_cnt and connect_fail_delay_time to config and let users decide how long they want to wait according to their network configuration?

[guolinke]

@StrikerRUS I think the exponential backoff strategy is better for the network connecting retry.
For the user-configurable, actually, I want to expose them for the first time. But then I think it is not a good idea, for we don't want to user to tune them.

And exponential backoff strategy should work for most cases:
for stable network env, it should connect successfully in a short time.
for unstable network env, it also provides enough time buffers to retry.

[StrikerRUS]

@guolinke Ah, got it! Thanks a lot for your detailed explanation!

[jameslamb]

Thanks for the changes and thorough explanation. Looks good!