auto_redirect should detect the change of a path alias's source

Problem/Motivation

The 'auto_redirect' config setting makes this module detect for a change in a path alias, and add a redirect for the old alias. For example:

1. There is a path alias on the site /myalias which points to /node/1.
2. This is changed so it's /myalias-new
3. redirect_path_alias_update() automatically creates a redirect from /myalias to /myalias-new.

However, it should also add a redirect when a path alias changes its source. For example:

1. There is a path alias on the site /myalias which points to /node/1.
2. This is changed so it points to node/2.
3. redirect_path_alias_update() should create a redirect from /node/1 to /myalias.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes