`@@dolt_transaction_commit` not persisted

```
% dolt sql
# Welcome to the DoltSQL shell.
# Statements must be terminated with ';'.
# "exit" or "quit" (or Ctrl-D) to exit.
persist> set @@persist.dolt_transaction_commit=1;
persist> set @@global.dolt_transaction_commit=1;
persist> exit
Bye
zachmu@zachmu-thinkpad:dolt-scratch/test/persist% dolt sql
# Welcome to the DoltSQL shell.
# Statements must be terminated with ';'.
# "exit" or "quit" (or Ctrl-D) to exit.
persist> select @@dolt_transaction_commit
-> ;
+-----------------------------------+
| @@SESSION.dolt_transaction_commit |
+-----------------------------------+
| 0 |
+-----------------------------------+
1 row in set (0.00 sec)
```

This error message is also bad:

```
select @@persist.dolt_transaction_commit=1;
unknown scope `GLOBAL, PERSIST` on system variable `dolt_transaction_commit`
```

MySQL error is more reasonable:

```
> select @@persist.max_connections;
ERROR 1193 (HY000): Unknown system variable 'persist.max_connections'
```