MB-7478: From master

Add the rebalancer interface from master
MB-7478: Pull in vbucket rebalancer

Pull in the rebalancer into ns_server as we do not want it in
a seperate repository
MB-7478: Add a temporary build

Build rebalancer from ns_server
MB-7478: Code hooked up, unit tests do not run yet
MB-7478: Hookup calls, tests do not run yet
MB-7478: Do not try to fix inputs

As bad inputs cause crash, disallow any input not completely
meeting assumptions of rest of the code
MB-7478: Reduce logging, track input/outut to file
MB-7478: Log inputs and outputs

Working but tests fail. Some inputs cause FPE
MB-7478: Merge branch '2.0.1'

Conflicts:
	src/mb_kang_map.erl