sql: use leased database descriptors, implement database schema changes

[thoszhang]

No description provided.

[cockroach-teamcity]

This change is 

[ajwerner]

Reviewed 1 of 6 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @ajwerner and @lucy-zhang)

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pkg/sql/catalog/descs/collection.go, line 216 at r2 (raw file):

Quoted 8 lines of code…

	mutDesc, ok := db.(*sqlbase.MutableDatabaseDescriptor)
	if !ok {
		// TODO (lucy): Here and elsewhere in the Collection, we return a nil
		// descriptor with a nil error if the type cast doesn't succeed, regardless
		// of whether flags.Required is true. This seems like a potential source
		// of bugs.
		return nil, nil
	}

I think if this is nil and the flag is required we should fatal as it is a programming error from the implementation of the physical accessor.

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pkg/sql/catalog/descs/collection.go, line 343 at r2 (raw file):

> // TODO (lucy): This logic is quite similar to GetObjectVersion. Think about
> // unifying once we also have support for schemas.
> ```
:pray: 

[asubiotto]

We need to get this PR in pretty soon as it is a multitenant MVP blocker (#52617)

[thoszhang]

Moved to #52975.