Consider making JobParameters#getParameters() immutable

[marschall]

JobParameters#getParameters() currently returns a mutable defensive copy of the job parameters, even though the Javadoc says

@return an unmodifiable map containing all parameters.

This means that anybody who just wants read-only access to the job parameters, eg. only to iterate over them, eg. SimpleJobRepository#createJobExecution has to incur the cost of a defensive copy of a LinkedHashMap. Copying a LinkedHashMap is unfortunately quite expensive in term of memory allocation.

It looks like to copy was made modifiable with BATCH-671 / #2905 / e68a413