Running on recent master, commit f1dbf92ff0475a01d20170ea422c1d086acbbc57

If I run:
```
test/functional/feature_fee_estimation.py --randomseed 108574360997204915
```

I get:
```
2020-12-19T20:14:06.175000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_bcwexp78
2020-12-19T20:14:07.738000Z TestFramework (INFO): This test is time consuming, please be patient
2020-12-19T20:14:07.738000Z TestFramework (INFO): Splitting inputs so we can generate tx's
2020-12-19T20:14:10.870000Z TestFramework (INFO): Finished splitting
2020-12-19T20:14:12.104000Z TestFramework (INFO): Will output estimates for 1/2/3/6/15/25 blocks
2020-12-19T20:14:12.104000Z TestFramework (INFO): Creating transactions and mining them with a block size that can't keep up
2020-12-19T20:14:15.879000Z TestFramework (INFO): Creating transactions and mining them at a block size that is just big enough
2020-12-19T20:14:21.104000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/sdaftuar/ccl-bitcoin/2020-12-latest-master/test/functional/test_framework/test_framework.py", line 126, in main
    self.run_test()
  File "test/functional/feature_fee_estimation.py", line 256, in run_test
    check_estimates(self.nodes[1], self.fees_per_kb)
  File "test/functional/feature_fee_estimation.py", line 140, in check_estimates
    check_smart_estimates(node, fees_seen)
  File "test/functional/feature_fee_estimation.py", line 130, in check_smart_estimates
    % (feerate, last_feerate))
AssertionError: Estimated fee (0.000565) larger than last fee (0.000479) for lower number of confirms

```
assert(confTarget > 0); 
    /** true is passed to estimateCombined fee for target/2 and target so
     * that we check the max confirms for shorter time horizons as well.
     * This is necessary to preserve monotonically increasing estimates.
     * For non-conservative estimates we do the same thing for 2*target, but
     * for conservative estimates we want to skip these shorter horizons
     * checks for 2*target because we are taking the max over all time
     * horizons so we already have monotonically increasing estimates and
     * the purpose of conservative estimates is not to let short term
     * fluctuations lower our estimates by too much.
     */
    double halfEst = estimateCombinedFee(confTarget/2, HALF_SUCCESS_PCT, true, &tempResult);
    if (feeCalc) {
        feeCalc->est = tempResult;
        feeCalc->reason = FeeReason::HALF_ESTIMATE;
    }
    median = halfEst;
    double actualEst = estimateCombinedFee(confTarget, SUCCESS_PCT, true, &tempResult);
    if (actualEst > median) {
        median = actualEst;
        if (feeCalc) {
            feeCalc->est = tempResult;
            feeCalc->reason = FeeReason::FULL_ESTIMATE;
        }
    }
    double doubleEst = estimateCombinedFee(2 * confTarget, DOUBLE_SUCCESS_PCT, !conservative, &tempResult);
    if (doubleEst > median) {
        median = doubleEst;
        if (feeCalc) {
            feeCalc->est = tempResult;