Add adjustable timeout setting

When trying to download a very large multipart file i would like to be able to adjust the timeout setting on the socket from the current default 5min.

eg:
--socket-timeout=30s

```
SOCKET_TIMEOUT = 5 * 60 # in sec(s) (timeout if we don't receive any recv() callback)
```

Locate SOCKET_TIMEOUT and the command-line option parsing, then trace the multipart download timeout path. Done means a --socket-timeout value such as 30s changes the current five-minute receive timeout while downloads continue to work with the default when no option is supplied.

Written by the indexing model from the issue text.