can't alter access_* settings

Nobody has claimed this yet.

In version `Manticore 3.4.3 d74eea9@200601 dev` I cannot alter index settings access_* :

```
mysql> show index t3 settings;
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| settings | |
+---------------+-------+
1 row in set (0.00 sec)

mysql> alter table t3 access_plain_attrs='mlock' access_blob_attrs='mlock';
Query OK, 0 rows affected (0.00 sec)

mysql> show index t3 settings;
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| settings | |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show create table t3;
+-------+----------------------------+
| Table | Create Table |
+-------+----------------------------+
| t3 | CREATE TABLE t3 (
f text
) |
+-------+----------------------------+
1 row in set (0.00 sec)
```

It's not convenient as requires full RT index rebuild if I just want to mlock it in memory.

Start by reproducing the reported SHOW INDEX, ALTER TABLE, and SHOW CREATE TABLE commands on an RT index using the referenced Manticore version or current behavior. Trace the ALTER TABLE handling for access_plain_attrs and access_blob_attrs. Done means these settings persist and are visible without requiring a full RT index rebuild.

Written by the indexing model from the issue text.