The group fingerprint (merkle root) showed as part of the group info is not correct if a member is removed in off-chain groups

[vplasencia]

Description

The group fingerprint (which is the merkle root) is not correct if a member is removed from an off-chain group. The issue is that if a member is removed, the member is removed from the Bandada infrastructure instead of converting it to 0 which is what the current IMT (Semaphore v3) does.

If the member is converted to 0 people can have off-chain and on-chain compatibility when using the Bandada fingerprint and Semaphore v3 on-chain.

When Semaphore v4 is included, this issue will still exist (#638).

Semaphore uses 0s to keep transactions on-chain cheaper. If the app is completely off-chain it's not necessary to use 0s but in case people want to use the group on-chain too, it will be necessary to have 0s for removed members.

Keeping the current implementation can benefit off-chain groups that are large and remove a lot of members frequently. But it's still better to have 0s for removed members to have compatibility on-chain.

If a member is removed from a group the fingerprint (tree root) changes because the cached group uses 0 for the removed member but the Bandada infrastructure removed the member (did not use 0 for the removed member). Then when the server is restarted, the tree created has a different root (group fingerprint) since the 0 of the member removed is not saved in the Banadada infrastructure. More context on this: bandada-infra/boilerplate#27