We discussed that the auto explain feature is rarely used.
This PR removes only the automatic explain. You can still display
the explain output for any given relation using `ActiveRecord::Relation#explain`.
As a side-effect this should also fix the connection problem during
asset compilation (#9385). The auto explain initializer in the `ActiveRecord::Railtie`
forced a connection.
The sample application with the Getting Started Guide was very out of
date. I've re-done it on edge (as of 51b9def5bf) so it should
be good to go with Rails 4. It's also in synch with what the guide
actually says.
