Bug description
In remote partitioning jobs which use the MessageChannelPartitionHandler with database polling, on each poll of the database where there is 1 or more new completed worker StepExecutions, the partition handler loads and keeps in memory an additional copy of the corresponding JobInstance, JobExecution, and all StepExecutions and their ExecutionContexts until the partition step is completed.

This leads to high memory consumption during the partition step and can lead to out of memory errors if the poll interval is short enough and the number of partitions is high enough, especially since the ExecutionContexts are held in memory as well.

Environment
Any environment using spring-batch-integration 5.0.1 and above (93800c6) which also uses the MessageChannelPartitionHandler with database polling.

Steps to reproduce
Run a remote partitioning batch job with database polling, a short poll interval, high number of partitions, and limited available memory.

Expected behavior
Polling of the database in remote partitioning jobs does not lead to constant gradual increase of consumed memory until the partition step completes.

Minimal Complete Reproducible example
Minimal Complete Reproducible example is here: spring-batch-mcve-memory-leak.zip

The example runs a remote partitioning batch job with 1000 partitions, each having an ExecutionContext containing a single UUID. In order to exacerbate the memory consumption, the poll interval is set to very low (2ms), and the worker step sleeps for 50ms before completing. This allows a new copy of the JobInstance, JobExecution, StepExecutions, and ExecutionContexts to be loaded and held in memory each time a worker step completes.

Please run the example with the -Xmx64m -XX:+HeapDumpOnOutOfMemoryError jvm options:

MAVEN_OPTS='-Xmx64m -XX:+HeapDumpOnOutOfMemoryError' mvn package exec:java -Dexec.mainClass=org.springframework.batch.MyBatchJobConfiguration

This should cause an OutOfMemoryError to be thrown rather quickly and the resulting heap dump should contain the following:

• 60-65 instances of JobInstance, JobParameters, JobExecition
• 60k-65k instances of StepExecution, ExecutionContext

Analysis
In MessageChannelPartitionHandler#pollReplies, the callback calls JobExplorer#getJobExecution. The SimpleJobExplorer implementation loads the JobExecution and all of the StepExecutions as well as their ExecutionContexts. Each of these StepExecutions also contains a reference to the JobExecution and thus to all other StepExecutions indirectly. If any of the loaded StepExecutions is completed and not present in the result Set, they are added to it, and this causes the currently loaded JobExecution instance and all of the other StepExecution instances to be held in memory.