Add retry logic to DB calls in case of failure due to temporary network problem

[bluerogue251]

I'm opening this issue to see how much demand there is for support for handling poor network conditions more gracefully.

Right now I believe a single network call failing results in the whole subsetting process failing. However, I haven't been able to reproduce poor network conditions myself, so it's been hard to test this.

I generally run DBSubsetter in AWS where network connections are very reliable, and my subsets generally take under 10 minutes, so it's not that big of a problem if they occasionally fail and need to be retried. However, if people use DBSubsetter in different conditions, this might not be the case.

If you would like such a feature please add a 👍 to this issue, and feel free to leave any specific notes about your use case in this thread.