Assertion failure in rebalancer_f after many ro->rw->ro transitions on a node

[sergepetrenko]

On vshard 0.1.27 this assertion fails after many ro->rw transitions:

vshard/vshard/storage/init.lua

Lines 2895 to 2897 in 2211be2

	local function rebalancer_f()
	assert(not M.rebalancer_service)
	local service = lservice_info.new('rebalancer')

[Serpentian]

Seems like a duplicate of https://github.com/tarantool/vshard-ee/issues/9