Global system variables don't update in other sessions

System variables that **only** exist at a global scope (i.e. not at global and session scopes) should have changes visible in all sessions immediately when they are updated. Dolt currently seems to make a session-scoped copy of the system variable when the session is created and the global value can only be seen when explicitly using the `GLOBAL` keyword.

## MySQL 8.0 Behavior
```
-- Session 1:
select @@gtid_purged;
+---------------+
| @@gtid_purged |
+---------------+
| |
+---------------+
1 row in set (0.00 sec)

-- Session 2:
set @@GLOBAL.gtid_purged='9765edf2-82d5-11f0-99b2-ce1c22533a29:1-2';
Query OK, 0 rows affected (0.00 sec)

-- Session 1:
select @@gtid_purged;
+------------------------------------------+
| @@gtid_purged |
+------------------------------------------+
| 9765edf2-82d5-11f0-99b2-ce1c22533a29:1-2 |
+------------------------------------------+
1 row in set (0.00 sec)
```

## Dolt 1.78.5 Behavior
```
-- Session 1:
select @@gtid_purged;
+---------------+
| @@gtid_purged |
+---------------+
| |
+---------------+
1 row in set (0.00 sec)

-- Session 2:
set @@GLOBAL.gtid_purged='9765edf2-82d5-11f0-99b2-ce1c22533a29:1-2';
Query OK, 0 rows affected (0.00 sec)

-- Session 1:
select @@gtid_purged;
+---------------+
| @@gtid_purged |
+---------------+
| |
+---------------+
1 row in set (0.00 sec)
```