Avoid automatically creating the vip_meta_key_value index on huge sites

[joshbetz]

I installed a large database in my local environment and the schema update that was triggered automatically caused some problems. In most cases the index will already exist, so it won't be a problem. If possible, we might want to avoid automatically creating this index on sites with many millions of postmeta entries though.

https://github.com/Automattic/vip-go-mu-plugins/blob/master/schema.php#L12-L26

[sjinks]

Related: #2893

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