partition table: always store offset

[frairon]

This PR removes the functionality of "store only newest offest" in the partition table during SetValue().

The idea of this limitation was to make sure we don't overwrite new partition-offsets with old values. However, a side effect was that if the offset was somehow corruped and higher than the actual offset, it would never get fixed.

The function was actually never necessary, because

• SetValues() are executed sequentially per partition
• --> the producer-emits are also pushed sequentially to the channel
• --> the offset-results will also be in order (i.e. monotonically increasing). The PR adds a system test to verify this.

This should also reduce some load from the storage, since it removes the extra GetOffset on every SetValue.