Limit on-demand census

[lucasmenendez]

Allows to a user to create a census of an strategy in a certain block number. It requires to get every posible holder balance for the desired block number.

This is posible after #137, it is because this feature changes how the scanner stores the holders, storing them with negativo or zero balance even if they are not holders any more.

This makes that whole the historical holders be saved, and making posible to get their balances on a certain block.

However this process is very intensive in terms of web3 requests, at least the number of historical holders.

[lucasmenendez]

Already implemented under debug API endpoints /holders/{tokenID} & /holders/queue/{queueID}.

These endpoints contain a testing porpoise and not optimised implementations.