Write-Only values don't send a value changed notification

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Write only values cannot request a new value since the device will not respond. 
But preventing the request value call also never allows a notification to be 
sent. The challenge is when to update the value. For listening devices, we can 
do so immediately but for sleeping devices the value won't be updated until 
some point in the future. The library needs a way to track when a set is 
completed so the write only value can then be "completed".

Original issue reported on code.google.com by glsatz on 21 Feb 2013 at 5:17