fix partitioner integer division incoherence + unit tests

There was a default in partitioning: when partition number was not a divider of the input prefixes map size, the aftermentioned formula was true, which we do not want: prefixesPerPartition * partitionNumber < totalPrefixes.

This MR aims to dispatch remaining prefixes equally to partitions at the end of their construction, so we have total prefixes number split in a most equal possible way.

Unit tests are mandatory.