Improve caching logic for http.send (#5316)

In order to calculate if a cached entry expired or not, we would previously
read the headers stored with the request, and parse each of the relevant date-related
ones and use those to compare with the current time. #4960 improved this by allowing
us to ignore the Date header completely when force_cache is enabled.

We now go one step further, and calculate and store an expiresAt value at the time the
entry is created. This allows for a much simpler check of expiry, and avoids re-parsing
the header values with each lookup.

Fixes #5300

Signed-off-by: Anders Eknert <anders@eknert.com>