Fix ThreadSanitizer data race in get_total_connection_errors

The ThreadSanitizer detected a data race where the main thread
was reading m_connection_errors from m_cur_stats and m_stats list
while worker threads were modifying them.

Fixed by:
1. Making get_total_connection_errors() read only from m_totals
   (similar to get_total_ops(), get_total_bytes(), etc.)
2. Adding totals::update_connection_error() method to update
   m_totals.m_connection_errors incrementally
3. Calling m_totals.update_connection_error() from
   run_stats::update_connection_error()

This ensures m_totals.m_connection_errors is updated atomically
and can be safely read from the main thread without data races.

Author: fcostaoliveira