message: Fix data race condition in BanMan::DumpBanlist()
  The m_is_dirty value being read in BannedSetIsDirty() can differ from
  the value being set in SweepBanned(), i.e., be inconsistent with a
  BanMan instance internal state.
diff: @@ -43,9 +43,11 @@ void BanMan::DumpBanlist()
  static Mutex dump_mutex;
  LOCK(dump_mutex);
  - SweepBanned(); // clean unused entries (if bantime has expired)
  -
  + {
  + LOCK(m_cs_banned);
  + SweepBanned();
  if (!BannedSetIsDirty()) return;
  + }
  int64_t n_start = GetTimeMillis();
  