[buffer] Skip pieces not used for turn-in-piece in a more early phase in
partition. This can in some cases enhance performance more than 40%
[buffer] remove now unused neighbour variable and remove now non-used code,
now that analysis is done more early
[partition] small change, resize vector and assign, instead of pushing back
[buffer] move skipping neighbours to separate method
[buffer] add monotonic properties per piece
[buffer] use monotonic properties to analyse monotonic-offsets more efficiently
(it does not increase performance in country_buffer testcase)
[buffer] fix typo in comment
[buffer] avoid calculating the box if not necessary