Make blackhole max index key length comparable to InnoDB

[adamcable]

Currently the maximum index key length for an InnoDB table is 3072 bytes.
We typically use the Blackhole storage engine on various replicas with low storage space, or other requirements.
The expectation is there that you should be able to run an ALTER TABLE ... ENGINE=BLACKHOLE; command without issues on any InnoDB table. However, if the index length is > 1000 bytes this fails.
This PR aligns the maximum key length with the InnoDB storage engine.

[mysql-oca-bot]

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[adamcable]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
Please make sure to include your MySQL bug system user (email) in the returned form.
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Yes - this has been done.

Thanks,
Adam

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
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[adamcable]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=103371 for updates.
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