Fix members tangle calculation when reset on new epochs

[Powersource]

Needed if things end up like suggested here ssbc/ssb-group-exclusion-spec#17 (comment)

[Powersource]

Maybe we could extract the getGroupInfo out of https://github.com/ssbc/ssb-tribes2/blob/master/lib/get-tangle.js and move it into https://github.com/ssbc/ssb-tribes2/blob/master/lib/add-tangles.js Then we could pass different roots in as arguments into the different GetTangles. Then get-tangle.js could be kept/made even more generic. Initially we could maybe just calculate the epoch root. If that turns out to be slow then we could maybe store it in the keyring.