LP#1646166 Hatch extension connect via DOM, remote cache

LP#1646166 Hatch extension connect via DOM, remote cache

Determine Hatch availability by checking for the presence of a
well-known Hatch attribute in the DOM, inserted into the page
by the extension at load time.

Cache get/set values when talking to the Hatch extension (on a per-page
basis) to reduce unnecessary Hatch I/O.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>