Use same SidebarHookHandlers instance during request

on wikipedia, it is called hundreds of times when we really
need and should use one instance.

 2.21% 9.216    196 - Wikibase\Client\Hooks\SidebarHookHandlers::newFromGlobalState

Change-Id: Ie8c86a733ab8352679688c515386d8ee5041a9b2
Merge "Use same SidebarHookHandlers instance during request"