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_rename.cc

@@ -37,7 +37,6 @@ bool mysql_rename_tables(THD *thd, TABLE_LIST *table_list, bool silent)
   TABLE_LIST *ren_table= 0;
   int to_table;
   char *rename_log_table[2]= {NULL, NULL};
-  int disable_logs= 0;
   DBUG_ENTER("mysql_rename_tables");
 
   /*
@@ -79,12 +78,6 @@ bool mysql_rename_tables(THD *thd, TABLE_LIST *table_list, bool silent)
                               ren_table->table_name_length,
                               ren_table->table_name, 1)))
       {
-        /*
-          Log table encoutered we will need to disable and lock logs
-          for duration of rename.
-        */
-        disable_logs= TRUE;
-
         /*
           as we use log_table_rename as an array index, we need it to start
           with 0, while QUERY_LOG_SLOW == 1 and QUERY_LOG_GENERAL == 2.
@@ -136,12 +129,6 @@ bool mysql_rename_tables(THD *thd, TABLE_LIST *table_list, bool silent)
                  rename_log_table[1]);
       DBUG_RETURN(1);
     }
-
-    if (disable_logs)
-    {
-      logger.lock();
-      logger.tmp_close_log_tables(thd);
-    }
   }
 
   pthread_mutex_lock(&LOCK_open);
@@ -200,13 +187,6 @@ bool mysql_rename_tables(THD *thd, TABLE_LIST *table_list, bool silent)
   pthread_mutex_unlock(&LOCK_open);
 
 err:
-  /* enable logging back if needed */
-  if (disable_logs)
-  {
-    if (logger.reopen_log_tables())
-      error= TRUE;
-    logger.unlock();
-  }
   start_waiting_global_read_lock(thd);
   DBUG_RETURN(error);
 }