fix Managed tokens added/removed

[mendesfabio]

After working on #538 I realized I missed to handle tokens added/removed. It's not critical for now as pools from Kassandra are only going to change weights (not add/remove tokens) but still something we should fix. There's this example query showing a pool that had its tokens composition changed and, because of that, some tokens don't have weight (see nulls).

We have to analyze this pool's events, ensure everything is being handled (also see if the order they're emitted makes sense), and finally call updatePoolWeights from either (or all, needs investigation) events handlers:

• handleTokenAdded
• handleTokenRemoved
• handleGradualWeightUpdateScheduled