topdown: Add retry option for http.send

When http.send reaches out to the server if an
entry is not found in the cache or a stale entry
needs to revalidated, a network error while
calling the server will result in http.send recording
the error in the cache. It may sometimes be useful to
retry the request in case of intermittent failures etc.

This change adds a new option to the http.send input
object which allows policy authors to specify a retry count
for executing a HTTP request. Retries are performed with
an exponential backoff delay.

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