Cover retry more places in code, implement exponential back off

[Slach]

[jkilpatrick-twilio]

We are running into some AWS rate limiting and some exponential backoff would really help us out. Just saying that in case it helps raising the priority on this for 2.7.0.

[Slach]

@jkilpatrick-twilio
you are welcome to make PR

and look documentation about concurrency in s3 config section and retry policy and upload_concurrency in general section
https://github.com/Altinity/clickhouse-backup/tree/master/ReadMe.md