z_mergetoaddress doesn't currently have any mechanism for specifying
the minconf value. We don't want to default back to a minconf of 1,
as before, because we'll be selecting anchors with at least 3
confirmations by default, so the simplest change here is to make
the minconf value default to the number of anchor confirmations, and
add a configurable minconf at some point in the future. It's likely
that `z_mergetoaddress` will end up being deprecated in any case,
so this works for now.
Also, ensure that anywhere that we're attempting to obtain the Orchard
anchor that we don't attempt to get an anchor from a height below NU5
activation.
Kris Nuttycombe
Kris Nuttycombe