ENG-5505: Remove unused partition count parameter from
Cartographer.computeReplacementPartitions().
ENG-5505: Use the cartographer to figure out the partitions to replace on
rejoin.

The topology may not reflect the real state of the cluster during
join. Rejoining node should ask the cartographer what partitions to replace.
Merge pull request #949 from VoltDB/ENG-5505

ENG-5505: Fix rejoin during rebalance