MINOR: cache: Add "Age" header.

MINOR: cache: Add "Age" header.

This patch makes the cache capable of adding an "Age" header as defined by
rfc7234.

During the storage of new HTTP objects we memorize ->eoh value and
the value of the "Age" header coming from the origin server.
These information may then be reused to return the cached HTTP objects
with a new "Age" header.

May be backported to 1.8.