ZIP based UDF dependency handling: smarter backoff instead of static sleep

[soxofaan]

Follow up of #845/#914

Because of the S3-FUSE-mount delay issues, I had to add a sleep of (now) 30s after putting the ZIP archive at /batch_jobs/{job_id}/udf-py-deps.zip, otherwise the executors didn't seem to see that file. Some warnings even seem to suggest that 30s is somtimes not enough.

Instead of a static sleep, it would be better to have a smarter, dynamic backoff strategy, with a couple of retries, and shorter sleeps (possibly exponential)