Make it the responsibility of the connection to hold onto an ARel visitor for generating SQL. This improves the code architecture generally, and solves some problems with marshalling. Adapter authors please take note: you now need to define an Adapter.visitor_for method, but it degrades gracefully with a deprecation warning for now.

Conflicts:

	activerecord/lib/active_record/relation/finder_methods.rb
	activerecord/test/cases/adapter_test.rb