BUG/MINOR: lua: fix extra 500ms added to socket timeouts

BUG/MINOR: lua: fix extra 500ms added to socket timeouts

Since commit #56cc12509, haproxy accepts double values for timeouts. The
value is then converted to  milliseconds before being rounded up and cast
to int. The issue is that to round up the value, a constant value of 0.5
is added to it, but too early in the conversion, resulting in an
additional 500ms to the value. We are talking about a precision of 1ms,
so we can safely get rid of this rounding trick and adjust resulting
timeouts equal to 0 to a minimum of 1ms.

This patch is specific to the 1.9 branch and doesn't require to be
backported.