[Swaps] Do something with the exit fee

[lsaether]

https://github.com/zeitgeistpm/zeitgeist/blob/4f3e71d641b45fa37cc1f05e0e60d8ac1876b36b/zrml/swaps/src/macros.rs#L36

When someone leaves a pool, they can get charged an exit_fee (currently set to 0 on the testnet). However, we don't do anything with this exit fee and should pay it somewhere.

[maltekliemann]

Bumping to next milestone where swaps will get a general makeover.