Add allow_retry kwarg to execute (7)

[andynu]

Add allow_retry kwarg to execute

This extends the existing class level auto_retry functionality to optionally allow an allow_retry boolean kwarg to allow retry of specific queries.

This PR preserves the requirement that autocommit? be enabled, which is the safer choice.

  def with_retry(allow_retry: false) # :nodoc:
    should_retry = (allow_retry || self.class.auto_retry?) && autocommit?

See issue #2310

Stacked on #2372