Warn about changing mirror_url

[fschulze]

Changing the mirror_url can lead to cases where the database is unexpectedly not updated anymore. People assume that this should just work, but when PyPI was used, the database has references to the serials of PyPI and if a different mirror doesn't send them, there won't be updates anymore due to serial mismatch errors.

This should be documented and there should be info on the +simple pages if this happens.

It might also be useful to be able to reset the mirror, so it works without serials from the new location without having to recreate the mirror index and loosing already downloaded releases.