Planner: start last slot early instead of late

[andig]

Currently, last slot is started as late as possible which has drawbacks:

• potentially introduces gaps in the charging plan
• if charging takes longer than projected, vehicle will not be fully charged

We can remove this special behaviour if we can ensure that slots are not too long (i.e. not longer than 1 hour). In this case there is no disadvantage if we start at the beginning of the slot.

[mhibu]

It's better to start charging at the beginning of the last slot.
Most times the charging lasts 15min longer because of heating the akku or slow charging because of cold akku.