MB-100: un-write-only-ify scripts/visualize-rebalance.rb

I've found previous version quite hard to understand and to
fix/extend. I belive that's likely due to "spread"-ness of state
there.

New code is "local-state" as much as possible with code flow
reflecting timeline state changes of every nodes.

Hopefully it will be much less write-only :)

Change-Id: I7f7a6a0b7192f04ec2197f0cf024af9129c411ad
Reviewed-on: http://review.couchbase.org/24576
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>