I wanted to open an issue to understand why `bnb_search` as [implemented](https://github.com/bitcoin/bitcoin/pull/10637) @achow101 does not terminate once an optimal search is found.  This behavior is evident I believe when inspecting the [test](https://github.com/bitcoin/bitcoin/blob/master/src/wallet/test/coinselector_tests.cpp#L178) which sets the target to `5` and uses the utxo pool `4,3,2,1`.  since combinations of `4` are evaluated first, the `bnb_search` finds an optimal solution of `4,1`.  However, instead of terminating when the `best_waste = 0` (no better solution can be found), the search continues and instead finds another solution `2,3` which also has a `best_waste = 0`.  Why not terminate once an optimal solution is found instead of continuing to search?
add_coin(2*CENT);           
        
        BOOST_CHECK(!testWallet.SelectCoinsMinConf( 3 * CENT, filter_standard, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 3 * CENT, filter_confirmed, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        BOOST_CHECK_EQUAL(nValueRet, 3 * CENT);
        add_coin(5*CENT);           
        add_coin(10*CENT, 3, true); 
        add_coin(20*CENT);          
        
        
        BOOST_CHECK(!testWallet.SelectCoinsMinConf(38 * CENT, filter_standard, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        
        BOOST_CHECK(!testWallet.SelectCoinsMinConf(38 * CENT, filter_standard_extra, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf(37 * CENT, filter_standard, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        BOOST_CHECK_EQUAL(nValueRet, 37 * CENT);
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf(38 * CENT, filter_confirmed, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        BOOST_CHECK_EQUAL(nValueRet, 38 * CENT);
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf(34 * CENT, filter_confirmed, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        BOOST_CHECK_EQUAL(nValueRet, 35 * CENT);       
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);     
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 7 * CENT, filter_confirmed, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));
        BOOST_CHECK_EQUAL(nValueRet, 7 * CENT);
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
        
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 8 * CENT, filter_confirmed, GroupCoins(vCoins), setCoinsRet, nValueRet, coin_selection_params, bnb_used));