The `max_past` parameter, supposed to stop endorsing of blocks that are
too old, compared absolute times rather than relative times.
Specifically, it would check whether the block timestamp or the current
time was more recent than 110 seconds after epoch.
The new version checks whether the difference between the block
timestamp and the current time is greater than 110seconds.
When removing old endorsments from the wallet (to avoid it growing),
only old endorsments were kept, or no endorsments at all were kept.
In addition, that function relied on the endorsments being sorted by
level.
