Options to set limit on number and size of packages to download.

[analog-cbarber]

It is pretty easy when mirroring from large repos to inadvertently get more than you really need.

One way to address this would be to add limits on both the number of packages and the total
size of packages to be downloaded and to not download anything if the limit is exceeded. We
can pick some suitable default value and require users to override it for larger mirroring jobs.

[dcstang]

Thanks, has this been implemented? Am running into space issues when mirroring Python3.