Merge #14582: wallet: always do avoid partial spends if fees are within a specified range

7f13dfb587 test: test the implicit avoid partial spends functionality (Karl-Johan Alm)
b82067bf69 wallet: try -avoidpartialspends mode and use its result if fees are below threshold (Karl-Johan Alm)

Pull request description:

  The `-avoidpartialspends` feature is normally disabled, as it may affect the optimal fee for payments. This PR introduces a new parameter `-maxapsfee` (max avoid partial spends fee) which acts on the following values:
  * -1: disable partial spend avoidance completely (do not even try it)
  * 0: only do partial spend avoidance if fees are the same or better as the regular coin selection
  * 1..∞: use APS variant if the absolute fee difference is less than or equal to the max APS fee

  For values other than -1, the code will now try partial spend avoidance once, and if that gives a value within the accepted range, it will use that.

  Example: -maxapsfee=0.00001000 means the wallet will do regular coin select, APS coin select, and then pick AKS iff the absolute fee difference is <= 1000 satoshi.

  Edit: updated this to reflect the fact we are now using a max fee.

ACKs for top commit:
  fjahr:
    tested ACK 7f13dfb587
  achow101:
    ACK 7f13dfb587
  jonatack:
    ACK 7f13dfb58, code review, debug build, verified the test fails with `AssertionError: not(2 == 1)` for the number of vouts when `-maxapsfee=0.0001` is changed to 0, and verified the new logging with an added assertion.
  meshcollider:
    Code review ACK 7f13dfb587

Tree-SHA512: 475929df57f6191bb4e36bfbcad5a280a64bb0ecd8767b76cb2e44e2301235d0eb294a3f2fac5bbf15d35d7ecfba47acb2285feadb883c9ce31c08377e3afb3c

src/wallet/init.cpp

@@ -49,6 +49,7 @@ void WalletInit::AddWalletOptions(ArgsManager& argsman) const
     argsman.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data. 0 to entirely disable the fallbackfee feature. (default: %s)",
                                                                CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     argsman.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u). Warning: Smaller sizes may increase the risk of losing funds when restoring from an old backup, if none of the addresses in the original keypool have been used.", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
+    argsman.AddArg("-maxapsfee=<n>", strprintf("Spend up to this amount in additional (absolute) fees (in %s) if it allows the use of partial spend avoidance (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_MAX_AVOIDPARTIALSPEND_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     argsman.AddArg("-maxtxfee=<amt>", strprintf("Maximum total fees (in %s) to use in a single wallet transaction; setting this too low may abort large transactions (default: %s)",
         CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MAXFEE)), ArgsManager::ALLOW_ANY, OptionsCategory::DEBUG_TEST);
     argsman.AddArg("-mintxfee=<amt>", strprintf("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)",