Spam attack on transfer_ins table

[sim31]

Contract the way it's done now could be attacked by creating bunch of small transfers to some account, to the point where that account couldn't claim any of his tokens.

This happens because mint action has to iterate through all the the rows in transfer_ins table in order to calculate the amount of tokens to be claimed. But this can take a lot of time and exceed the max transaction time, if table contains a lot of rows. Attacker could take advantage of that by spamming the table with small transfers.

One potential solution is to create a new action which could claim for each transfer_in individually.