Remove call to getHeadNodeState in LockBasedScheduler constructor

When creating the ChangeDispatcher instance in the constructor of LockBasedScheduler we should avoid the call to LockBasedScheduler.getHeadNodeState(). This is a call to an overridable method at construction time, which can be troublesome as the instance in not fully initialised yet. On top of that the default implementation might have side effects through its call to refreshHead. We should probably just use head.get() to initialise that ChangeDispatcher instance.