Cluster: configEpoch collisions resolution.

The slave election in Redis Cluster guarantees that slaves promoted to
masters always end with unique config epochs, however failures during
manual reshardings, software bugs and operational errors may in theory
cause two nodes to have the same configEpoch.

This commit introduces a mechanism to eventually always end with different
configEpochs if a collision ever happens.

As a (wanted) side effect, this also ensures that after a new cluster
is created, all nodes will end with a different configEpoch automatically.
Cluster: redis-trib cluster allocation more even across nodes.

redis-trib used to allocate slots not considering fractions of nodes
when computing the slots_per_node amount. So the fractional part was
carried over till the end of the allocation, where the last node
received a few more slots than any other (or a lot more if the cluster
was composed of many nodes).

The computation was changed to allocate slots more evenly when they are
not exactly divisible for the number of masters we have.
Cluster: CLUSTER SETSLOT implementation comment updated.

Update the comment since the implementation details changed.