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

```
: m_estimation_filepath{estimation_filepath}
{
    static_assert(MIN_BUCKET_FEERATE > 0, "Min feerate must be nonzero");
    size_t bucketIndex = 0;
    for (double bucketBoundary = MIN_BUCKET_FEERATE; bucketBoundary <= MAX_BUCKET_FEERATE; bucketBoundary *= FEE_SPACING, bucketIndex++) {
        buckets.push_back(bucketBoundary);
        bucketMap[bucketBoundary] = bucketIndex;
    }
    buckets.push_back(INF_FEERATE);
    bucketMap[INF_FEERATE] = bucketIndex;
    assert(bucketMap.size() == buckets.size());
    feeStats = std::unique_ptr<TxConfirmStats>(new TxConfirmStats(buckets, bucketMap, MED_BLOCK_PERIODS, MED_DECAY, MED_SCALE));
    shortStats = std::unique_ptr<TxConfirmStats>(new TxConfirmStats(buckets, bucketMap, SHORT_BLOCK_PERIODS, SHORT_DECAY, SHORT_SCALE));
    longStats = std::unique_ptr<TxConfirmStats>(new TxConfirmStats(buckets, bucketMap, LONG_BLOCK_PERIODS, LONG_DECAY, LONG_SCALE));
    AutoFile est_file{fsbridge::fopen(m_estimation_filepath, "rb")};
    if (est_file.IsNull()) {
        LogPrintf("%s is not found. Continue anyway.\n", fs::PathToString(m_estimation_filepath));
        return;
    }
    std::chrono::hours file_age = GetFeeEstimatorFileAge();
    if (file_age > MAX_FILE_AGE && !read_stale_estimates) {
        LogPrintf("Fee estimation file %s too old (age=%lld > %lld hours) and will not be used to avoid serving stale estimates.\n", fs::PathToString(m_estimation_filepath), Ticks<std::chrono::hours>(file_age), Ticks<std::chrono::hours>(MAX_FILE_AGE));
        return;
    }
    if (!Read(est_file)) {
        LogPrintf("Failed to read fee estimates from %s. Continue anyway.\n", fs::PathToString(m_estimation_filepath));
    }
}
CBlockPolicyEstimator::~CBlockPolicyEstimator() = default;