(from discussion on IRC. Currently a global `settxfee` has to be changed to override the fee rate, it would make sense to be able to specify it on a per-transaction basis. This would continue #7518)

```
-*- jonasschnelli thinks we should probably add a optional feerate parameter to fundrawtransaction
<wumpus> jonasschnelli: there isn't one yet?
-*- jonasschnelli looking..
<wumpus> I'm surprised, would be so much better than changing a global
<jonasschnelli> no.. i don't think so. Maybe there is a PR.
<sipa> there is a pr
<jonasschnelli> Hard to keep track of everything. :)
<wumpus> no, the PR does it wrong
```

Regarding implementation:

```
<jonasschnelli> The ugly thing is, that CreateTransaction uses the static DEFAULT_TARGET
<wumpus> passing in an optional feerate would be the obvious thing to do
<jonasschnelli> So, this would require some refactoring.
<sipa> just add a confirmation target to coincontrol
<jonasschnelli> Yes. This sounds good.
<wumpus> right, just slap it into coincontrol
<sipa> wasn't there a PR that added a bunch of options to fundrawtransaction?
<sipa> including that
<jonasschnelli> Not the feerate.
<jonasschnelli> But the rest is merged.
```

mutable CAmount nCreditCached;
    mutable CAmount nImmatureCreditCached;
    mutable CAmount nAvailableCreditCached;
    mutable CAmount nWatchDebitCached;
    mutable CAmount nWatchCreditCached;
    mutable CAmount nImmatureWatchCreditCached;
    mutable CAmount nAvailableWatchCreditCached;
    mutable CAmount nChangeCached;
    CWalletTx()
    {
        Init(NULL);
    }
    CWalletTx(const CWallet* pwalletIn)
    {
        Init(pwalletIn);
    }
    CWalletTx(const CWallet* pwalletIn, const CMerkleTx& txIn) : CMerkleTx(txIn)
    {
        Init(pwalletIn);
    }
    CWalletTx(const CWallet* pwalletIn, const CTransaction& txIn) : CMerkleTx(txIn)
    {
        Init(pwalletIn);
    }
    void Init(const CWallet* pwalletIn)
    {
        pwallet = pwalletIn;
        mapValue.clear();
        vOrderForm.clear();
        fTimeReceivedIsTxTime = false;