Fee payment prices fetched on demand

[mrq1911]

Currently we fetch all payment assets prices on initialize regardless if they are used or not. This is not very efficient, especially with a growing number of assets.

HydraDX-node/pallets/transaction-multi-payment/src/lib.rs

Line 81 in f617f3c

fn on_initialize(_n: T::BlockNumber) -> Weight {

Price should be populated only if requested by any transaction.