The feerate security margin is a multiplicative factor applied to the
feerate of some transactions in order to guarantee that the
transaction remains publishable and has a sufficient chance of being
confirmed, that we can base some of our decisions on that.
The multiplicative factor is >=1 and was so far a constant 2. This
might have been sensible in the low-fee environment, where the fees
are expected to oscillate, and almost guaranteeing that we will
eventually have rising feerates but in high-fee environments that is
no longer the case, and the 100% margin that the multiplicator 2
brings is excessive. We therefore opt to start out with 100%, then
linearly interpolate up to a given maxfeerate (which does not have to
be a real feerate ever reached, it just indicates the feerate after
which we apply the constant 10% margin.
Fixes#6974Closes#6976
[Fixed up all the other changes required, including spendable calcualtion
comments and unit test and pytest tests --RR]
24ec17126c