Metadata update issues

[firthm01]

Found some more issues uncovered by fixing #99 (PR #111 )

• Some input plugin parameters are not setting the "changed" flag to true, meaning they are not affecting the audible output because the monitoring plugins ignore the metadata.

• New monitoring plugins initially do not render existing inputs correctly until a parameter is changed. This is because the "changed" flag in the metadata from these input is false and so the monitoring plugin does not process it. However, the monitoring plugin must still process it if it has never seen metadata for a particular input before, because it needs to set the correct initial state for that input.

These issues were initially hidden by #99, because the changed flag was never reset, and so ALL metadata was reported as changed and ALL metadata messages were therefore processed.

[rsjbailey]

extra ; here

[rsjbailey]

I'd be a bit tempted to factor all these out into something that wraps the mutex and the set_changed call as it'd be really easy to miss either if modifying/updating this.

something like

// object_metadata_sender.hpp
class ObjectMetadataSender {
...
private:
template<typename F>
    auto setData(F f) -> decltype(f(data_)) {
        std::lock_guard<std::mutex> lock(dataMutex_);
        data_.set_changed(true);
        return f(data_.mutable_obj_metadata());
    }
};
// object_metadata_sender.cpp
void ObjectMetadataSender::range(float value) {
  setData([value](auto data){ data->set_range(value); });
}
// etc..

Which is a variation of this. Could simplify it a bit if nothing returns a value (just have setData return void and drop the auto/trailing return type/decltype). Might not need the trailing type in any case if we're on C++17.

I dunno, maybe changing two repeated lines with one slightly more complicated one is overkill, what do you reckon?