Badly-defined behavior calling `put` reusing existing key.

[rainwoodman]

The current behavior seems to be replacing the value of the key, but increases total_bytes without retiring the existing item.

I do not think this behavior is well defined. It causes total_bytes and available_bytes to diverge, and eventually triggers #15.

How unique do we expect the keys to be?

Shall we raise an exception if the key same key is used twice, or shall we consistently retire the existing key/value when the same key is reused?

[rainwoodman]

Seems to be triggered by https://github.com/dask/dask/pull/4460