Apply RetryOnConflict on Bulk documents

[csabavirag]

This PR is created to solve the issue on the missing RetryOnConflict (retry_on_conflict) metadata attribute when adding documents in bulk (detailed at #2182)

[ruflin]

Thanks for the PR. Change LGTM. Could you add an entry to the changelog? And can you check in the test suite if there is an easy way to add a test for this?

Normally we make changes to 8.x first and then backport to 7.x. But currently there is quite a large PR ongoing against 8.x to make it 8 compatible: #2181 What I propose is to get this in but in parallel follow up with a PR against the 8.x branch to ensure we don't forget about it. Otherwise I worry we will miss this feature in 8.x.

[ruflin]

@csabavirag I did just trigger the test suite and it seems to break one of the tests. It seems that in some cases the object is null so you need to add a check for this.

[csabavirag]

I pushed a new commit with does not set RetryOnConflict if it has already been set on the document/script. I know retry_on_conflict is stripped from the final message(s) if its value is 0, but thought to exclude this case in order to a smaller memory footprint.
This allowed me to run BulkTest.php successfully. Only 1 error was reported, but I believe this is not related to these changes

Output of make docker-run-phpunit PHPUNIT_OPTIONS="tests/BulkTest.php":

There was 1 failure:

1) Elastica\Test\BulkTest::testMemoryUsage
Failed asserting that 2.3990734932979025 is less than 1.31.

I also updated the BulkTest suite to check the parameter is set on the document/script properly when only the client has retry_on_conflict set ("global option").

[ruflin]

Change LGTM, waiting on CI to run through.

[ruflin]

Should we put this under "fixed"? sounds more like a bug fix but also happy to keep it under Added.

[csabavirag]

First I added to that section. Then I moved to Added. TBH, I would leave it up to you.

[ruflin]

Lets keep it, I can always clean it up later.

[ruflin]

@csabavirag Thank you for the contribution!

Could you follow up with a PR against 8.x. The big 8.x change made it in and we should make sure this change also makes it into 8.x