It encapsulates the logics of a JoinRequest batch processing.

Topology module will be quite important in picodata. This first version
misses a lot of features, but a few commits later it's going to
implement quite a lot of logics.

When a new instance is joined - there's one complex thing: raft leader
has to decide where this new instance is going to be emplaced, i.e. what
replicaset should it join. There're many different parameters have an
influence - `repliction_factor`, `failure-domain`, and of course the
existing topology. So, this new `topology` module must make the decision.

This patch only refactors the current Picodata behavior, and doesn't
bring new features for its users. Instead, it opens the door to a future
development.

Also, this patch provides a unit-testing basis for the future features.