Limit daily gas consumption for AMB

[akolotov]

At this particular moment there are no daily limits for the bridge operations in the AMB mode. The current limitation is only for maximum gas usage per call. It means that the user-contracts could send as many requests as they want. In order to reduce the risk of the DDoS attacks on the bridge validators it is suggested to introduce the following limits:

• maximum daily gas usage per user-contract. The limit is set as one per bridge contract but will be calculated separately for each user-contract. If the limit is zero it will also allow to stop the bridge operations.
• relay requests rate per user-contract. We can monitor number of failed requests (most probably it is an attempt to dry out validators accounts) originated by every user-contract and manage their rate by increasing the value of suggested parameter. E.g. 0 means that there is no limit on rate, 1 is for 1 request per block for particular user-contract, 10 corresponds to 1 request per 10 blocks for particular user-contract.