exit-only rollup account

[krlosMata]

Summary

exit-only rollup accounts accumulate all the transactions received in exitBalance. In order to detect this new type of account, we set a special babyjubjub, 0x000...0001, to identify this accouunt type.

Brief description

• rollup-tx.circom: needs to detect only-exit accounts and increase exitBalance instead of balance
• rollup-tx-states.circom: add isOnlyExit signal to check account type. This signal would be used afterwards in balance-updater.circom to select the proper balance to update

[krlosMata]

implemented in https://github.com/hermeznetwork/circuits/pull/48