message: Add readlock when accessing nSequenceId
  Need a lock on cs_mapBlockIndex when accessing nSequenceId
diff: @@ -148,9 +148,11 @@ bool CParallelValidation::Initialize(const boost::thread::id this_id, const CBlo
  // Assign the nSequenceId for the block being validated in this thread. cs_main must be locked for lookup on
  // pindex.
  + {
  + READLOCK(cs_mapBlockIndex);
  if (pindex->nSequenceId > 0)
  pValidationThread->nSequenceId = pindex->nSequenceId;
  -
  + }
  pValidationThread->fIsValidating = true;
  }
  