A request debug mode

Provide a mechanism to produce request traces to a path user specified.
I'm thinking of something similar to 'tcpflow -e http' . It should probably store one request per file, and let file names contain some connection id and a timestamp. Users can know what happens in which order within a connection.

IMHO this should be global and/or local:

Local (log specific requests):

```
use Hijk;
use Hijk::DEBUG trace => "/tmp/hijk_debug/";
Hijk::DEBUG::request({...}); # logged
Hijk::request({...}); # not logged
```

Global (log all requests):

```
use Hijk;
use Hijk::DEBUG trace => "/tmp/hijk_debug/", global => 1;
Hijk::DEBUG::request({...}); # logged
Hijk::request({...}); # logged
```