Fixed erroneous handling of "paginated" items in the dashboard's timeline

[JBBianchi]

In the Dashboard timeline, the streams are read per chucks of 100 items. When the length to be read was set over 100, there were two "paging" issues:

• gathering of duplicate events for each new chunk (off-by-one error: the last item of the previous chunk was gathered again as the first item of the next chunk due to a bad computation of the offset)
• length overflow (when requesting 101 items, 200 items were gathered instead because of a lack of proper computing for the length of the last chunk)

This PR then provides two fixes:

• Proper computing of the start offset of the extra chunks
• Proper computing of the length of the last chunk

Note:
In addition, due to (I guess) a change in the deserialisation of CloudEvents, when trying to read the sequence attribute, a Decimal is returned instead of a ulong before. This case has been added to CloudEventExtensions