Cleanup of slave code:

- Added testing if connection is killed to shortcut reading of connection data
  This will allow us later in 10.2 to do a cleaner shutdown of slaves (less errors in the log)
- Add new status variables: Slaves_connected, Slaves_running and Slave_connections.
- Use MYSQL_SLAVE_NOT_RUN instead of 0 with slave_running.
- Don't print obvious extra warnings to the error log when slave is shut down normally.

sql/sql_parse.cc

@@ -1177,6 +1177,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
 #ifdef HAVE_REPLICATION
   case COM_REGISTER_SLAVE:
   {
+    status_var_increment(thd->status_var.com_register_slave);
     if (!register_slave(thd, (uchar*)packet, packet_length))
       my_ok(thd);
     break;