run the link checks in parallel
run the link checks in parallel

Use a ThreadPool to check the links in parallel so the check runs
faster. Results are still reported in order.

Also only get the HEAD and not the whole body of the page.

Change-Id: Ie4fbb97dd3393ac050b36057aee3b939dea58de2
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals