Make the Splitter default allocation be factory-based

[nventuro]

The ProtocolFeeSplitter currently has a single default fee allocation. This is too coarse, it's unlikely the current behavior would ever be used as Pools are too different.

Ideally, we'd be able to set different default fees for each Pool factory. The trouble here is that we'd then need to find the factory for each Pool whenever tokens are withdrawn, but the list of factories should not be very large (likely ~5), so this would not be an issue.