QLowEnergyCharacteristic - cached values (OS X and iOS)

Cache a characteristic's value in update/write notification only
if characteristic has a 'Read' property.

Change-Id: Idc705e0c4f2ee704a80bf37b4e59e1dd09ca9e4f
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>