topdown: Address negative duration for the current age of http response

The current age of a http response is calculated as the difference
between the current time and the value contained in the "Date" response
header. There are couple of scenarios that could lead to the current age
being represented as a negative duration.

1. Since the value of "Date" response header is parsed using Go's
time.Parse method, it does not contain a monotonic clock reading. As a result,
the time.Sub method uses wall clock readings to determine the difference between
current time and the parsed version of the response time.

2. The server could set a value for the "Date" response header which may not be a true indication of
when the response was generated.

This change updates the logic that determines whether a cached response is fresh or not,
to treat the resposne as stale if the current response age is represented as a negative duration.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>