Replace weight argument in xtokens::transfer with max_fee

[sander2]

Related to #656.

In practice, I believe people just put some arbitrary high amount for the weight in xtokens::transfer. To me it does make sense to have this as an argument at all. Users do not care how much weight is used on the destination chain - it is simply a representation of computation time on the remote chain. As such, it makes more sense to me to either have the weight argument be optional, or to remove it altogether. If not supplied, the code would set the limit to Unlimited.

If anything, users may care about how many fees they pay. So we could consider adding an optional max_fee argument to xtokens::transfer. Note that the BuyExecution instruction takes both a max_fee and a max_weight argument. However, even the use max_fee is dubious. If the fee is insufficient, presumably the transferred tokens get trapped and the user can then recover them using a ClaimAsset instruction. However, the user would still end up paying the fees to pay for execution of this new message..

Interested in hearing your thoughts about this.

[shaunxw]

Agreed. The weight parameter was designed when there was no way to specify it as unlimited. I think it's time to move on and make it an optional weight_limit. For most cases, front-end apps can pass none to indicate unlimited. In the case similar to limited_reserve_transfer_assets in pallet-xcm, some weight limit can be provided.