Use Overpass epoch offset for adiff seq id

[CloudNiner]

As described at:
https://wiki.openstreetmap.org/wiki/Overpass_API/Augmented_Diffs#Time_slices_and_numbering

Now we generate the correct diff seq id for the end timestamp of the diff:

OSM Change Sequence Id 4199848 @ 2020-09-17 18:48:03+00:00 -> Minutely Augmented Diff Id 4215592
Augmented Diff written to: Bucket <bucket>, Path: onramp-diffs/004/215/592.xml.gz (gzip=True)
Augmented diff 4215592 generated in 2.12628436088562s
Committed: 4199847 -> 4199848 in 0.054 seconds.

If I pull the Overpass diff for the same sequence, I now get mostly the same elements as in the Onramp diff:

4215592.zip

Closes #41

[CloudNiner]

I think the original bug here was because I pulled the seq id to timestamp formula from https://github.com/geotrellis/vectorpipe. I've opened an issue there as well geotrellis/vectorpipe#142