Fix query cache to load before first request

[eileencodes]

In a test app we observed that the query cache was not enabled on the
first request. This was because the query cache hooks are installed on
load and active record is loaded in the middle of the first request.

If we remove the on_load from the railtie the query cache hooks will
be installed before the first request, allowing the cache to be enabled
on that first request.

This is ok because query cache doesn't load anything else, only itself
so we're not eager loading all of active record before the first
request, just the query cache hooks.

[Eileen M. Uchitelle & Matthew Draper]

[matthewd]

I just checked the other places we install an executor/reloader hook inside an on_load, and those are okay:

rails/actionview/lib/action_view/railtie.rb

Lines 77 to 81 in 2a470d7

	ActiveSupport.on_load(:action_view) do
	unless ActionView::Resolver.caching?
	app.executor.to_run ActionView::Digestor::PerExecutionDigestCacheExpiry
	end
	end

rails/actioncable/lib/action_cable/engine.rb

Lines 73 to 75 in 2a470d7

	app.reloader.before_class_unload do
	ActionCable.server.restart
	end

Those are both [only] arranging to clear out state that the relevant component may have accumulated, so it's fine to not invoke them if is hasn't been loaded yet.

This is the only case where we're setting state around the request, and therefore need it to always be present.

[rafaelfranca]

Should not those hooks run on boot time in production? All models are loaded so the model that is loaded would execute the on_load hook.

[matthewd]

@rafaelfranca yeah, this should only manifest when eager loading isn't on, so it's fairly low impact, but it's still a bug.