(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.
```

{
        
        set< set<CTxDestination>* > hits;
        map< CTxDestination, set<CTxDestination>* >::iterator it;
        BOOST_FOREACH(CTxDestination address, grouping)
            if ((it = setmap.find(address)) != setmap.end())
                hits.insert((*it).second);
        
        set<CTxDestination>* merged = new set<CTxDestination>(grouping);
        BOOST_FOREACH(set<CTxDestination>* hit, hits)
        {
            merged->insert(hit->begin(), hit->end());
            uniqueGroupings.erase(hit);
            delete hit;
        }
        uniqueGroupings.insert(merged);
        
        BOOST_FOREACH(CTxDestination element, *merged)
            setmap[element] = merged;
    }
    set< set<CTxDestination> > ret;
    BOOST_FOREACH(set<CTxDestination>* uniqueGrouping, uniqueGroupings)