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

```
#include <policy/fees.h>
#include <clientversion.h>
#include <common/system.h>
#include <consensus/amount.h>
#include <kernel/mempool_entry.h>
#include <logging.h>
#include <policy/feerate.h>
#include <primitives/transaction.h>
#include <random.h>
#include <serialize.h>
#include <streams.h>
#include <sync.h>
#include <tinyformat.h>
#include <uint256.h>
#include <util/fs.h>
#include <util/serfloat.h>
#include <util/time.h>
#include <algorithm>
#include <cassert>
#include <chrono>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <exception>
#include <stdexcept>
#include <utility>
static constexpr double INF_FEERATE = 1e99;
std::string StringForFeeEstimateHorizon(FeeEstimateHorizon horizon)