Support multiple buckets for meta tiles

[zerebubuth]

Follows on from tilezen/tilequeue#353.

For better resiliency, we should support writing to multiple buckets. This would mean handling the pass-through of that configuration from the CodeBuild job, and making sure that the metatile-exists check is run on the last bucket in the list (so that it repeats jobs for which only some prefix of the list of buckets was written to).

[zerebubuth]

Fixed in #40 and tilezen/tilequeue#356.