Generate less timeouts on pages that make a lot of API requests

 * Increase maxSockets for API requests. The default in node is 5 per host.
   Parsing pages with lots of templates makes many concurrent connections
   that end up timing out. Bumping this up shaves at least a minute off
   that large test case in the bug report:

   fr/Liste_des_premiers_préfets_français_(1800-1815)

 * Cache and coalesce ImageInfoRequests.

Bug: 55585
Change-Id: Ia025a90a52b45c0816983d1b7b34fc074b6b4c4f