Support for custom connection attributes

[dveeden]

This makes it possible to send custom connection attributes to the server.
Enhances #343

mysql [localhost] {msandbox} (performance_schema) > select * from session_connect_attrs where processlist_id<>connection_id();
+----------------+--------------+-----------------+------------------+
| PROCESSLIST_ID | ATTR_NAME    | ATTR_VALUE      | ORDINAL_POSITION |
+----------------+--------------+-----------------+------------------+
|            125 | _pid         | 8481            |                0 |
|            125 | _platform    | amd64           |                1 |
|            125 | program_name | test2           |                2 |
|            125 | cattr1       | foo             |                3 |
|            125 | cattr2       | bar             |                4 |
|            125 | _os          | darwin          |                5 |
|            125 | _client_name | Go MySQL Driver |                6 |
+----------------+--------------+-----------------+------------------+
7 rows in set (0.00 sec)

[julienschmidt]

Honestly, I don't see any need for setting custom attributes.
Unless someone can convince me otherwise, let's focus on #343