fix amendment: AMM swap should honor invariants: (#5002)

The AMM has an invariant for swaps where:
new_balance_1*new_balance_2 >= old_balance_1*old_balance_2

Due to rounding, this invariant could sometimes be violated (although by
very small amounts).

This patch introduces an amendment `fixAMMRounding` that changes the
rounding to always favor the AMM. Doing this should maintain the
invariant.

Co-authored-by: Bronek Kozicki
Co-authored-by: thejohnfreeman

src/test/jtx/impl/AMM.cpp

@@ -234,7 +234,6 @@ AMM::expectBalances(
         balances(asset1.issue(), asset2.issue(), account);
     return asset1 == asset1Balance && asset2 == asset2Balance &&
         lptAMMBalance == STAmount{lpt, lptIssue_};
-    return false;
 }
 
 IOUAmount