MDEV-33582 Add more warnings to be able to better diagnose network issues

Warnings are added to net_server.cc when
global_system_variables.log_warnings >= 4.

When the above condition holds then:
- All communication errors from net_serv.cc is also written to the
  error log.
- In case of a of not being able to read or write a packet, a more
  detailed error is given.

Other things:
- Added detection of slaves that has hangup to Ack_receiver::run()
- vio_close() is now first marking the socket closed before closing it.
  The reason for this is to ensure that the connection that gets a read
  error can check if the reason was that the socket was closed.
- Add a new state to vio to be able to detect if vio is acive, shutdown or
  closed. This is used to detect if socket is closed by another thread.
- Testing of the new warnings is done in rpl_get_lock.test
- Suppress some of the new warnings in mtr to allow one to run some of
  the tests with -mysqld=--log-warnings=4. All test in the 'rpl' suite
  can now be run with this option.
 - Ensure that global.log_warnings are restored at test end in a way
   that allows one to use mtr --mysqld=--log-warnings=4.

Reviewed-by: <serg@mariadb.org>,<brandon.nesterenko@mariadb.com>

sql/semisync_master_ack_receiver.cc

@@ -149,7 +149,7 @@ bool Ack_receiver::add_slave(THD *thd)
   slave->thd= thd;
   slave->vio= *thd->net.vio;
   slave->vio.mysql_socket.m_psi= NULL;
-  slave->vio.read_timeout= 1;
+  slave->vio.read_timeout= 1;                   // 1 ms
 
   mysql_mutex_lock(&m_mutex);
 
@@ -337,6 +337,17 @@ void Ack_receiver::run()
         */
         net.compress= slave->thd->net.compress;
 
+        if (unlikely(listener.is_socket_hangup(slave)))
+        {
+          if (global_system_variables.log_warnings > 2)
+            sql_print_warning("Semisync ack receiver got hangup "
+                              "from slave server-id %d",
+                              slave->server_id());
+          it.remove();
+          m_slaves_changed= true;
+          continue;
+        }
+
         len= my_net_read(&net);
         if (likely(len != packet_error))
         {