There are some potential feefilter cleanups mentioned and motivated in comment https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-805808485 . If you address the locking one, please be careful and make sure that `round()` is only called under a lock. See https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-807992199

#### Useful skills:

* Bitcoin protocol messages
* Bitcoin Core P2P design
* Running our unit and functional tests

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#include <node/utxo_snapshot.h>
#include <policy/feerate.h>
#include <protocol.h> 
#include <script/script_error.h>
#include <sync.h>
#include <txmempool.h> 
#include <txdb.h>
#include <versionbits.h>
#include <serialize.h>
#include <util/check.h>
#include <util/hasher.h>
#include <util/translation.h>
#include <atomic>
#include <map>
#include <memory>
#include <optional>
#include <set>
#include <stdint.h>
#include <string>
#include <thread>
#include <utility>
#include <vector>
class CChainState;
class BlockValidationState;
class CBlockIndex;
class CBlockTreeDB;
class CBlockUndo;
class CChainParams;
struct CCheckpointData;
class CInv;