search: proper Cache and TTL info in HTTP headers

• BETTER The search results pages emits proper Cache and TTL information in its HTTP headers, so that any eventual external cachers (such as varnish) could act accordingly to invalidate their caches automatically, without any configuration. (closes #2302)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>