This adds a Rack middleware and Sidekiq middleware for detecting
"Cross-database modification" violations. These occur when you query 2
different databases in the context of a single transaction. Previously
we only wrapped our RSpec code with this detection logic but we found
that it was often causing lots of false positives in RSpec (in test-only
code) which made it hard to sort through the real issues. In addition we
want extra validation by running this in production and therefore
detecting code that our specs may be missing.

The middlewares use a feature flag `detect_cross_database_modification`
which we intend to enable for a small percentage of time.

The middlewares can also be disabled by setting the env var
`DISABLE_CROSS_DATABASE_MODIFICATION_DETECTION=true` and restarting the
application. This environment variable is an extra safety net if the
middlewares are causing problems.