Closes #5202 and #919

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.

This fixes CVE-2011-0448

otherwise when using external ActiveRecord adapters (e.g. Oracle) database_statements might be loaded before active_support/core_ext/module/deprecation which results in NoMethodError (commit 60cf65de added "deprecate" call to database_statements.rb)

[#2991]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Revert "Temporarily revert "Update after_commit and after_rollback docs and tests to use new style API with an :on options instead of on_* suffix." and "Add after_commit and after_rollback callbacks to ActiveRecord that are called after transactions either commit or rollback on all records saved or destroyed in the transaction.""

This reverts commit 1b2941cb.

[#2991]