Reduce linear time complexity of transactions

[geigerzaehler]

The number of storage reads for the handler of the RegisterUser message is linear in the number of users. Similarly the number of reads for the UnregisterUser message is linear in the number of orgs and requires a number of comparisons linear in the size of all members of orgs.

These complexity characteristics may have a drastic effect on the incentives and prices in the network and may open the door for DoS attacks. Moreover fixing this will likely be very invasive. (For example it may require additional on-chain or off-chain storage.) This means that this problem needs to be solved earlier than later.

The first step would be to come up with potential solutions using on-chain and off-chain storage that reduce the complexity to sub-linear.