When an event is rolled back, we want to ensure that when it is retried
it maintains its priority (as represented by its uuid).  This commit,
on rollback, takes the rolled-backed event's uuid and creates a
replacement event that has the correct priority.
Restructured retry test to ensure that can force a retry from an
external file if concurrently run long_write_long and write_long_long
(with long_write_long having a higher priority).  Currently, retry
tests fail because not catching backout exception.
Was not catching backout exceptions (or any exceptions) thrown before
an endpoint or partner sequence call.  This commit handles that.
Added a test to check basic retries work.