Improve working with replication servers which only keep part of the history

[lonvia]

The geofabrik and openstreetmap.fr replication severs only keep diffs for the last couple of months. This can lead to strange behaviour when trying to download older diffs or trying to update an older extract.

This PR adds a couple of changes to improve handling with these kind of servers:

• New parameter for timestamp_to_sequence to refuse returning sequence numbers for timestamps older than the oldest available change.
• collect_diffs() now errors out when the first diff to be downloaded returns a 4xx error. Before it wold just signal that not all diffs could be downloaded, sending the caller potentially in an infinite retry loop.
• pyosmium_get_changes and pyosmium_up_to_date will now detect when diffs are no longer available and properly error out.

Fixes #257.