In writing PR #1408, it was not clear how to best return the balances of an account from the underlying database in a way that was useful for both the mempool and the query logic. The mempool needs a map and the query needs a sorted object with the goal of reduced allocations for both.

@SuperFluffy thinks writing an async stream would fix both

(will add code pointers after PR lands)

┆Issue Number: ENG-788