Continuing seeding fails with geopackages

[jsvrcek]

There is a bug when trying to reseed geopackage caches using a progress file.

When trying to reseed using the progress file, the seeder will try to get the tile timestamp from load_tile_metadata, which currently returns None. This throws a type error because None cannot be compared to int.

Expected Behavior

The expected behavior would be that reseeding a geopackage tile cache would work the same way as MBTiles.

Actual Behavior

It blows up.

Possible Fix

#386

Steps to Reproduce

1. Begin to seed data to a geopackage using a progress file.
2. Stop the seed halfway through.
3. Try to resume the seed job.

Context

This will affect anyone trying to continue seeding a geopackage.

Your Environment

• Version used: 1.11.0
• Environment name and version (e.g. Python 2.7.5 with mod_wsgi 4.5.9): Python 3.5.5
• Operating System and version: Ubuntu 14.04