Adjust latency extension requirements

[Trinitou]

proposal after some discussion on Dischord.

1. Someone reported that Bitwig and Reaper immediately call plugin_latency->get after host_latency->changed gets called from the plugin during activation
   • solution: allow plugin_latency->get to be called already during plugin->activate
2. I think it does not make sense if host_latency->changed is called before activate.
   • solution: require host_latency->changed to be called only during plugin->activate
   • open question: should it be mandatory for the plugin to call host_latency->changed during activate after setting up its latency internally? (How do existing hosts handle the case that plugins do not call host_latency->changed during activation? Maybe you can tell how Bitwig handles it, @abique?)

Not sure whether the term being-activated is optimal. If you have better suggestions, please feel free to change it or suggest something else.