Modify VariableSource from subscription

[tomlokhorst]

Figure out what to do when a source is modified from within a callback, when there's still more callbacks to process.

[mac-cain13]

A first version to prevent this from creating time consuming bugs could be:

• When we start to call the handlers set a flag, unset the flag when all handlers are called
• If the value is modified while the flag is set, fatalError

I think a fatalError is appropriate since it currently gives unpredictable behaviour and therefore is unsupported by this library.