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

```
}
            std::unique_ptr<TxConfirmStats> fileFeeStats(new TxConfirmStats(buckets, bucketMap, MED_BLOCK_PERIODS, MED_DECAY, MED_SCALE));
            std::unique_ptr<TxConfirmStats> fileShortStats(new TxConfirmStats(buckets, bucketMap, SHORT_BLOCK_PERIODS, SHORT_DECAY, SHORT_SCALE));
            std::unique_ptr<TxConfirmStats> fileLongStats(new TxConfirmStats(buckets, bucketMap, LONG_BLOCK_PERIODS, LONG_DECAY, LONG_SCALE));
            fileFeeStats->Read(filein, nVersionThatWrote, numBuckets);
            fileShortStats->Read(filein, nVersionThatWrote, numBuckets);
            fileLongStats->Read(filein, nVersionThatWrote, numBuckets);
            
            
            buckets = fileBuckets;
            bucketMap.clear();
            for (unsigned int i = 0; i < buckets.size(); i++) {
                bucketMap[buckets[i]] = i;
            }
            
            feeStats = std::move(fileFeeStats);
            shortStats = std::move(fileShortStats);
            longStats = std::move(fileLongStats);
            nBestSeenHeight = nFileBestSeenHeight;
            historicalFirst = nFileHistoricalFirst;
            historicalBest = nFileHistoricalBest;
        }
    }
    catch (const std::exception& e) {
        LogPrintf("CBlockPolicyEstimator::Read(): unable to read policy estimator data (non-fatal): %s\n",e.what());
        return false;
    }
    return true;
}
void CBlockPolicyEstimator::FlushUnconfirmed()