WL#3984 (Revise locking of mysql.general_log and mysql.slow_log)

Bug#25422 (Hang with log tables)
Bug 17876 (Truncating mysql.slow_log in a SP after using cursor locks the
          thread)
Bug 23044 (Warnings on flush of a log table)
Bug 29129 (Resetting general_log while the GLOBAL READ LOCK is set causes
           a deadlock)

Prior to this fix, the server would hang when performing concurrent
ALTER TABLE or TRUNCATE TABLE statements against the LOG TABLES,
which are mysql.general_log and mysql.slow_log.

The root cause traces to the following code:
in sql_base.cc, open_table()
  if (table->in_use != thd)
  {
    /* wait_for_condition will unlock LOCK_open for us */
    wait_for_condition(thd, &LOCK_open, &COND_refresh);
  }
The problem with this code is that the current implementation of the
LOGGER creates 'fake' THD objects, like
- Log_to_csv_event_handler::general_log_thd
- Log_to_csv_event_handler::slow_log_thd
which are not associated to a real thread running in the server,
so that waiting for these non-existing threads to release table locks
cause the dead lock.

In general, the design of Log_to_csv_event_handler does not fit into the
general architecture of the server, so that the concept of general_log_thd
and slow_log_thd has to be abandoned:
- this implementation does not work with table locking
- it will not work with commands like SHOW PROCESSLIST
- having the log tables always opened does not integrate well with DDL
operations / FLUSH TABLES / SET GLOBAL READ_ONLY

With this patch, the fundamental design of the LOGGER has been changed to:
- always open and close a log table when writing a log
- remove totally the usage of fake THD objects
- clarify how locking of log tables is implemented in general.

See WL#3984 for details related to the new locking design.

Additional changes (misc bugs exposed and fixed):

1)

mysqldump which would ignore some tables in dump_all_tables_in_db(),
 but forget to ignore the same in dump_all_views_in_db().

2)

mysqldump would also issue an empty "LOCK TABLE" command when all the tables
to lock are to be ignored (numrows == 0), instead of not issuing the query.

3)

Internal errors handlers could intercept errors but not warnings
(see sql_error.cc).

4)

Implementing a nested call to open tables, for the performance schema tables,
exposed an existing bug in remove_table_from_cache(), which would perform:
  in_use->some_tables_deleted=1;
against another thread, without any consideration about thread locking.
This call inside remove_table_from_cache() was not required anyway,
since calling mysql_lock_abort() takes care of aborting -- cleanly -- threads
that might hold a lock on a table.
This line (in_use->some_tables_deleted=1) has been removed.

sql/sql_delete.cc

@@ -906,9 +906,7 @@ bool mysql_truncate(THD *thd, TABLE_LIST *table_list, bool dont_send_ok)
   char path[FN_REFLEN];
   TABLE *table;
   bool error;
-  uint closed_log_tables= 0, lock_logger= 0;
   uint path_length;
-  uint log_type;
   DBUG_ENTER("mysql_truncate");
 
   bzero((char*) &create_info,sizeof(create_info));
@@ -960,18 +958,6 @@ bool mysql_truncate(THD *thd, TABLE_LIST *table_list, bool dont_send_ok)
       DBUG_RETURN(TRUE);
   }
 
-  log_type= check_if_log_table(table_list->db_length, table_list->db,
-                               table_list->table_name_length,
-                               table_list->table_name, 1);
-  /* close log tables in use */
-  if (log_type)
-  {
-    lock_logger= 1;
-    logger.lock();
-    logger.close_log_table(log_type, FALSE);
-    closed_log_tables= closed_log_tables | log_type;
-  }
-
   // Remove the .frm extension AIX 5.2 64-bit compiler bug (BUG#16155): this
   // crashes, replacement works.  *(path + path_length - reg_ext_length)=
   // '\0';
@@ -997,14 +983,6 @@ end:
     VOID(pthread_mutex_lock(&LOCK_open));
     unlock_table_name(thd, table_list);
     VOID(pthread_mutex_unlock(&LOCK_open));
-
-    if (opt_slow_log && (closed_log_tables & QUERY_LOG_SLOW))
-      logger.reopen_log_table(QUERY_LOG_SLOW);
-
-    if (opt_log && (closed_log_tables & QUERY_LOG_GENERAL))
-      logger.reopen_log_table(QUERY_LOG_GENERAL);
-    if (lock_logger)
-      logger.unlock();
   }
   else if (error)
   {