[#MDEV-21294] Name or content of metadata_lock

Either the content or the name of the column THREAD_ID in table information_schema.metadata_lock_info is wrong. Correct name would be CONNECTION_ID or SESSION_ID because content is the connection_id and not the thread id.

Proof:
1. Provoke a MDL:
1a) start transaction; select * from test where id < 10;
1b) alter table test add column c9 int;

SELECT * FROM information_schema.metadata_lock_info;

+-----------+-------------------------+---------------+----------------------+--------------+------------+

| THREAD_ID | LOCK_MODE               | LOCK_DURATION | LOCK_TYPE            | TABLE_SCHEMA | TABLE_NAME |

+-----------+-------------------------+---------------+----------------------+--------------+------------+

|       413 | MDL_BACKUP_ALTER_COPY   | NULL          | Backup lock          |              |            |

|       414 | MDL_SHARED_READ         | NULL          | Table metadata lock  | test         | test       |

|       413 | MDL_SHARED_UPGRADABLE   | NULL          | Table metadata lock  | test         | test       |

|       413 | MDL_INTENTION_EXCLUSIVE | NULL          | Schema metadata lock | test         |            |

+-----------+-------------------------+---------------+----------------------+--------------+------------+

SELECT *

    ->   FROM performance_schema.threads

    ->  WHERE THREAD_ID in (413, 414);

Empty set (0.001 sec)

SELECT *   FROM performance_schema.threads  WHERE processlist_ID in (413, 414);

+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------------------+------------------+------+--------------+

| THREAD_ID | NAME                      | TYPE       | PROCESSLIST_ID | PROCESSLIST_USER | PROCESSLIST_HOST | PROCESSLIST_DB | PROCESSLIST_COMMAND | PROCESSLIST_TIME | PROCESSLIST_STATE               | PROCESSLIST_INFO                   | PARENT_THREAD_ID | ROLE | INSTRUMENTED |

+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------------------+------------------+------+--------------+

|       429 | thread/sql/one_connection | FOREGROUND |            413 | root             | localhost        | test           | Query               |               41 | Waiting for table metadata lock | alter table test add column c9 int |                1 | NULL | YES          |

|       430 | thread/sql/one_connection | FOREGROUND |            414 | root             | localhost        | test           | Sleep               |               51 | NULL                            | NULL                               |                1 | NULL | YES          |

+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------------------+------------------+------+--------------+

2 rows in set (0.001 sec)

[MDEV-21294] Name or content of metadata_lock_info table is wrong Created: 2019-12-11 Updated: 2020-04-12
Status:	Open
Project:	MariaDB Server
Component/s:	Plugins
Affects Version/s:	10.4.8
Fix Version/s:	10.4

[MDEV-21294] Name or content of metadata_lock_info table is wrong Created: 2019-12-11 Updated: 2020-04-12