message: banman: add thread annotations and mark members const where possible
  Also remove misleading comment. ClearBanned is used by rpc as well.
diff: @@ -41,7 +41,7 @@ public:
  BanMan(fs::path ban_file, CClientUIInterface* client_interface, int64_t default_ban_time);
  void Ban(const CNetAddr& netAddr, const BanReason& banReason, int64_t bantimeoffset = 0, bool sinceUnixEpoch = false);
  void Ban(const CSubNet& subNet, const BanReason& banReason, int64_t bantimeoffset = 0, bool sinceUnixEpoch = false);
  - void ClearBanned(); // needed for unit testing
  + void ClearBanned();
  bool IsBanned(CNetAddr netAddr);
  bool IsBanned(CSubNet subNet);
  bool Unban(const CNetAddr& netAddr);
  @@ -57,12 +57,12 @@ private:
  //!clean unused entries (if bantime has expired)
  void SweepBanned();
  - banmap_t m_banned;
  CCriticalSection m_cs_banned;
  - bool m_is_dirty;
  + banmap_t m_banned GUARDED_BY(m_cs_banned);
  + bool m_is_dirty GUARDED_BY(m_cs_banned);
  CClientUIInterface* m_client_interface = nullptr;
  CBanDB m_ban_db;
  - int64_t m_default_ban_time;
  + const int64_t m_default_ban_time;
  };
  extern std::unique_ptr<BanMan> g_banman;
  