replication: use 'score' to find a join-master

The patch refactors the algorithm of finding a join-master (in
replicaset_find_join_master()) to use scores instead of multiple
iterations with different criteria.

The original code was relatively fine as long as it had only
one parameter to change - whether should it skip
`box.cfg{read_only = true}` nodes.

Although it was clear that it was "on the edge" of acceptable
complexity due to a second non-configurable parameter whether a
replica is in read-only state regardless of its config.

It is going to get more complicated when the algorithm will take
into account the third parameter whether an instance is
bootstrapped.

Then it should make decisions like "among bootstrapped nodes try
to prefer instances not having read_only=true, and not being in
read-only state". The easiest way to do so is to use
scores/weights incremented according to the instance's parameters
matching certain "good points".

Part of #5613