detect storage leaks automatically in unit tests

[brenzi]

We've had several storage leak issues in the past and I think we don't approach this systematically yet. We might catch most probable leaks if we

1. measure state size
2. register and grow a community with more ceremonies than reputation lifetime
3. purge community
4. measure state size again during same phase. assert equal to above q.e.d.

[brenzi]

https://substrate.stackexchange.com/questions/6099/how-to-test-for-storage-leaks-in-pallets

[clangenb]

I think it will probably be hard to test this because I assume there are other storages that might persist in non-encointer pallets, like timestamp etc, this would basically imply that we need to do some stuff manually again. However, this would at least be false positives and not false-negatives.