Changing from being the number of seconds to a time.Duration.
For backward compatibility, the configuration with the number
of seconds is still accepted but an error message is printed
asking the user to convert to duration.
Resolves#487
We use a hardcoded value of 2 seconds for Write deadline when
writing data to client's socket.
This PR makes that value configurable.
Question is should we push the setting down to the client's object
to avoid indirection such as client.srv.opts.WriteDeadline?
