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

```
}
    }
    if (!inBlock && (unsigned int)blocksAgo >= scale) { 
        assert(scale != 0);
        unsigned int periodsAgo = blocksAgo / scale;
        for (size_t i = 0; i < periodsAgo && i < failAvg.size(); i++) {
            failAvg[i][bucketindex]++;
        }
    }
}
bool CBlockPolicyEstimator::removeTx(uint256 hash, bool inBlock)
{
    LOCK(m_cs_fee_estimator);
    return _removeTx(hash, inBlock);
}
bool CBlockPolicyEstimator::_removeTx(const uint256& hash, bool inBlock)
{
    AssertLockHeld(m_cs_fee_estimator);
    std::map<uint256, TxStatsInfo>::iterator pos = mapMemPoolTxs.find(hash);
    if (pos != mapMemPoolTxs.end()) {
        feeStats->removeTx(pos->second.blockHeight, nBestSeenHeight, pos->second.bucketIndex, inBlock);
        shortStats->removeTx(pos->second.blockHeight, nBestSeenHeight, pos->second.bucketIndex, inBlock);
        longStats->removeTx(pos->second.blockHeight, nBestSeenHeight, pos->second.bucketIndex, inBlock);
        mapMemPoolTxs.erase(hash);
        return true;
    } else {
        return false;
    }
}
CBlockPolicyEstimator::CBlockPolicyEstimator(const fs::path& estimation_filepath, const bool read_stale_estimates)