wallet: try -avoidpartialspends mode and use its result if fees are below threshold

The threshold is defined by a new max avoid partial spends fee flag, which defaults to 0 (i.e. if fees are unchanged, use the grouped option).

src/dummywallet.cpp

@@ -35,6 +35,7 @@ void DummyWalletInit::AddWalletOptions(ArgsManager& argsman) const
         "-discardfee=<amt>",
         "-fallbackfee=<amt>",
         "-keypool=<n>",
+        "-maxapsfee=<n>",
         "-maxtxfee=<amt>",
         "-mintxfee=<amt>",
         "-paytxfee=<amt>",