Add Online DDL support to dbDelta

[pento]

Since MySQL 5.1, a bunch of table level operations can be done in an asynchronous manner.

We can add support for this by adding ALGORITHM=INPLACE to the ALTER TABLE query, but the fun part is that this algorithm isn't supported for all types of operations.

So, we have a couple of options:

• Keep an array of what operations can be done online in each MySQL and MariaDB version, which will need to be updated for each new version of MySQL.
• Try all ALTER queries with the algorithm flag, then catch any SQL errors and fall back to the old style if needed. This will need careful testing with old versions of MySQL, particularly.

​https://dev.mysql.com/doc/refman/5.7/en/innodb-create-index-overview.html

[chriscct7]

@pento should this be punted, maybe early, to a future release?

[pento]

Yeah, I didn't get around to writing this.

[pbearne]

In core, the only time we ALTER TABLEs is in upgrade.php, so we could just hand-code the ALGORITHM=INPLACE call (after a version check)

I am not sure we need code in the WPDB class to handle this.

Can we close this as won't fix?