the following code is deprecated:

Thread.new { Post.find(1) }.join

It should be changed to close the database connection at the end of
the thread:

Thread.new {
  Post.find(1)
  Post.connection.close
}.join

Only people who spawn threads in their application code need to worry
about this change.

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.

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.

- Make sure ConnectionManagement clears connections after each call, except in tests
Signed-off-by: NJoshua Peek <josh@joshpeek.com>