BUG: GH4080, int conversion of underlying series to float needs updating in parent frame

closes #4080

essentially a variation on #3970

here block consolidation and cache is not the issue,
rather the dtype of the series is int, and as a result of the update (directly on the series)
changes dtype to float (which is fine)

BUT the parent still has this in an Int Block

this PR essentially creates an Observer Pattern thru a weakref
that allows notification of chanes (propogated back to the BlockManager)
which can then take action if needed