In order to make the Map based JobRepository threadsafe, the
JobExecutino was modified to use a `CopyOnWriteArraySet` for the
collection backing the child `StepExecution` collection. However, this
collection option has bad performance characteristics when used with
large collections. When using partitioning, it can be common to have a
large number of `StepExecution` instances to add which drastically hurts
start up time.
This commit remove the use of the `CopyOnWriteArraySet` for the
`JobExecution#stepExecutions` and replaces it with a `LinkedHashSet`
wrapped via `Collection.synchronizedSet`.
Resolves BATCH-2384
fa286dc1e9