The updateable_value and updateable_value_source classes allow broadcasting
configuration changes across the application. The updateable_value_source class
represents a value that can be updated, and updateable_value tracks its source
and reflects changes. A typical use replaces "uint64_t config_item" with
"updateable_value<uint64_t> config_item", and from now on changes to the source
will be reflected in config_item. For more complicated uses, which must run some
callback when configuration changes, you can also call
config_item.observe(callback) to be actively notified of changes.
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