Source column defaults from its own table

[morozov]

Change summary:

1. Instead of sourcing column defaults from the from/online table, source them from its own table.
2. Add some more assertions that test the inverse diff. Those assertions would fail without the fix so I believe they cover the originally reported scenario.

Fixes #5322.

[morozov]

The expectation that there should be a diff is invalid. It's a bug that is mitigated down the line:

dbal/src/Platforms/AbstractMySQLPlatform.php

Lines 544 to 551 in a18ffc1

	// Don't propagate default value changes for unsupported column types.
	if (
	$columnDiff->hasChanged('default') &&
	count($columnDiff->changedProperties) === 1 &&
	($columnArray['type'] instanceof TextType || $columnArray['type'] instanceof BlobType)
	) {
	continue;
	}

We may try removing this condition as well.

[morozov]

Funny thing is that this is one of the conditions that cause #4747.