MySQL: read parameter 'path' (socket) in the YAML config

The `connectInfo` record (inherited from `Database.MySQL.Base`) has a field `connectPath`. I supposed that `Database.Persist.loadConfig` would parse the YAML field "path" like "host" and "port", but it silently ignores it. This simple patch allows to configure a socket access to a MySQL-compatible server (Sphinx Search in my case).

I suggest cherry-picking it to persistent-mysql-1.*.