Feature/gas subsidies

[mprishchepo]

This feature allows SFC to implement gas subsidies functionality on go-opera. 'Sponsor' account pays the gas fees when user interacts with a 'Nominee' contract address.

It contains methods for:

• adding a 'Sponsor' address for a given 'Nominee' address with a specified gas amount
• removing a 'Sponsor' address for a given 'Nominee' address
• getter that returns first 'Sponsor' address with enough amount of gas from the list of sponsors for the specified 'Nominee' address

[quan8]

is there any bound for the array?

[mprishchepo]

The Sponsor array is unbounded. To save a gas and avoid reaching block's gas limit we only loop through the whole array inside view functions. All state changing operations use indexes to modify the array.