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

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

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#include <policy/fees.h>
#include <policy/policy.h>
#include <primitives/block.h>
#include <primitives/transaction.h>
#include <random.h>
#include <reverse_iterator.h>
#include <scheduler.h>
#include <streams.h>
#include <sync.h>
#include <tinyformat.h>
#include <txmempool.h>
#include <txorphanage.h>
#include <txrequest.h>
#include <util/check.h> 
#include <util/strencodings.h>
#include <util/system.h>
#include <validation.h>
#include <algorithm>
#include <memory>
#include <optional>
#include <typeinfo>
static constexpr auto RELAY_TX_CACHE_TIME = 15min;
static constexpr auto UNCONDITIONAL_RELAY_DELAY = 2min;
/** Headers download timeout.
 *  Timeout = base + per_header * (expected number of headers) */
static constexpr auto HEADERS_DOWNLOAD_TIMEOUT_BASE = 15min;
static constexpr auto HEADERS_DOWNLOAD_TIMEOUT_PER_HEADER = 1ms;
/** Protect at least this many outbound peers from disconnection due to slow/
 * behind headers chain.
 */