Currently the appside's mempool does not take into account if accounts have the proper balance to pay for all of the transactions in their pending queue. The ill effects of this are currently mitigated by throwing out all transactions for an account if they have a failing transaction, but it would be better to not feed these too-low-balance transactions into the block building process if possible.

Having the pending queue be aware of the cost of the transaction in it's pending structure, and having the under-balance transactions instead be in the parked queue, would be better.

┆Issue Number: ENG-683