GH-4708 : in multi-threaded step, enforce the first chunk to be written by first thread

[glelarge]

Fix for #4708

In a muti-threaded step, chunks are written regardless the order of items read from input.
If the output format has separator between records (as for JSON), a format issue appear if the the first chunk it not written first.

Wrong result
Sometime the produced JSON is misformatted :

[
,
 {"code":10002,"ref":"B2C3D4E5F6G7","type":11,"nature":6,"etat":2,"ref2":"B2C3D4E5F6G7"} {"code":10001,"ref":"A1B2C3D4E5F6","type":10,"nature":5,"etat":1,"ref2":"A1B2C3D4E5F6"},
 {"code":10003,"ref":"C3D4E5F6G7H8","type":12,"nature":7,"etat":3,"ref2":"C3D4E5F6G7H8"},
 {"code":10004,"ref":"D4E5F6G7H8I9","type":13,"nature":8,"etat":4,"ref2":"D4E5F6G7H8I9"}
]

Expected result

[
 {"code":10001,"ref":"A1B2C3D4E5F6","type":10,"nature":5,"etat":1,"ref2":"A1B2C3D4E5F6"},
 {"code":10002,"ref":"B2C3D4E5F6G7","type":11,"nature":6,"etat":2,"ref2":"B2C3D4E5F6G7"},
 {"code":10003,"ref":"C3D4E5F6G7H8","type":12,"nature":7,"etat":3,"ref2":"C3D4E5F6G7H8"},
 {"code":10004,"ref":"D4E5F6G7H8I9","type":13,"nature":8,"etat":4,"ref2":"D4E5F6G7H8I9"}
]

Run tests with :

mvn -pl spring-batch-infrastructure -Dtest=TaskExecutorRepeatTemplateFirstChunkTests test

[fmbenhassine]

Closing this PR, as discussed in #4708 (comment).

Thank you for your time and effort anyway, much appreciated!