Potential improvements for the new x/gasless module

[zale144]

Technical debt in x/gasless

There are places where things could be simplified, written a bit more clearly, methods that are a bit too long with high cognitive complexity, making them a bit hard to interpret, such as:

• `x/gasless/keeper/fee_helper.go - size and cognitive complexity of methods should be reduced
• x/gasless/keeper/store.go - handling the closing of the iterator, such as in IterateAllTxGTIDs should be done simply by defer iter.Close()
• there are places where slices are initialized as literals. we should avoid this
• x/gasless/keeper/store.go/UpdateConsumerAllowance - Is this meant to update all the consumers with matching gasTank.Id? In that case the method should be named something like UpdateConsumerAllowances. Also, there are multiple places where consumptions are iterated to find the one with matching gasTank.Id. There should be a method for that.

There should be more unit tests across the module.

Potential future functionalities

• Currently we can only have one fee denom per gas tank. It would be nice to remove that restriction in the future.
• Another limitation is the possibility of having only one message per transaction. That can also be a bit restricting