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

```
{
    try {
        LOCK(m_cs_fee_estimator);
        fileout << 149900; 
        fileout << CLIENT_VERSION; 
        fileout << nBestSeenHeight;
        if (BlockSpan() > HistoricalBlockSpan()/2) {
            fileout << firstRecordedHeight << nBestSeenHeight;
        }
        else {
            fileout << historicalFirst << historicalBest;
        }
        fileout << Using<VectorFormatter<EncodedDoubleFormatter>>(buckets);
        feeStats->Write(fileout);
        shortStats->Write(fileout);
        longStats->Write(fileout);
    }
    catch (const std::exception&) {
        LogPrintf("CBlockPolicyEstimator::Write(): unable to write policy estimator data (non-fatal)\n");
        return false;
    }
    return true;
}
bool CBlockPolicyEstimator::Read(AutoFile& filein)