The `eql?` override has been added in very early commits but was
actually not used except in a test. It also caused performance problems
since each call to `eql?` would issue two database queries. A developer
would unknowingly trigger these when using `exchanges.uniq`. A mistake
that could have happened again in the future.
I moved the implementation to the test that was actually using it and
made a second test a bit more explicit.
73372a58ef