Bandersnatch

This is release is massively supported by **dstufft** getting bandersnatch
back in sync with current packaging ecosystem changes. All clap your hands
now, please.

- Refactor the generation update code to avoid weird update paths
due to, well, my personal kink: 'over complication'.

- Generate the simple index ourselves instead of copying it from PyPI.

- Support files hosted on a separate domain.

- Implement PEP 503 normalization rules while also providing support
for legacy and very legacy clients.

- Fix a long standing, misunderstood bug: a non-deleting mirror would
delete packages if they were fully removed from PyPI. `Fixes 61`

- Don't require a X-PyPI-Last-Serial header on file downloads.
(Thanks to **dstufft**.)

- Increase our generation to help mirrors recover potential
setuptools corruption after some data bug on PyPI.

- Fixes 54 by reordering the simple index page and file fetching
parts. Thanks **dstufft** for the inspiration.

- Stop syncing serversig files and even start removing them.

- Create a new generation to enforce a full sync when upgrading.
This is required to get the canonical names for all packages.

- Implement canonical package directory names to support an upcoming PIP
release and other tools. (Thanks to **dstufft**)

- Fix a race condition where workers could get stuck indefinitely waiting for
another item in a depleted queue. (Thanks to **hongqn**)