Allow multiple (and concurrent) Polls

[ctrlc03]

Currently in MACI, it is not possible to re use the same Maci contract for multiple polls due to the AccQueue structure inner working. After a merge operation, the index would skip ahead to the smallest subtree capacity, and this would not be replicated offchain causing state root mismatches. As the modification would include quite some changes to the logic, and these structures might be removed soon from MACI, we should look into fixing this in another way.

A proposed solution is to use a LazyIMT structure https://github.com/privacy-scaling-explorations/zk-kit/blob/main/packages/imt.sol/contracts/LazyIMT.sol which is cheap to use and does not require any merge operation.