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DB schema generated by a migration may look different in
rails 4 and 5 (because rails 5 may use different default values).
For this reason it's important to explicitly set for which rails
version a migration was written for.

See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912

it will decide the method for disable statement_timeout upon
per transaction or per session, based on how it's called.

When calling with a block, block will be executed and it will use
session based statement_timeout, otherwise will default to existing
behavior.