explorer: Add flipBalanceUpdate function
explorer: Add flipBalanceUpdate function

Having this as a separate function, with a comment, is so much easier
to read for someone maintaining the code.

input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype