Improve user experience of the Updater by changing order of operations (WIP/RFC)

[joshtrichards]

• Some steps are more likely to take longer than others
• Some steps make others unnecessary if they fail (e.g. if the update can't be downloaded or extracted)

e.g.

1. checkForExpectedFilesAndFolders()
2. checkWritePermissions()
3. downloadUpdate()
4. verifyIntegrity()
5. extractDownload()
6. createBackup() <---- moves here from step 3
7. setMaintenanceMode()
8. replaceEntryPoints()
9. deleteOldFiles()
10. moveNewVersionInPlace()
11. setMaintenanceMode
12. finalize

The current order is as follows:

updater/index.php

Lines 1335 to 1372 in 4d660d8

	switch ($step) {
	case 1:
	$updater->checkForExpectedFilesAndFolders();
	break;
	case 2:
	$updater->checkWritePermissions();
	break;
	case 3:
	$updater->createBackup();
	break;
	case 4:
	$updater->downloadUpdate();
	break;
	case 5:
	$updater->verifyIntegrity();
	break;
	case 6:
	$updater->extractDownload();
	break;
	case 7:
	$updater->setMaintenanceMode(true);
	break;
	case 8:
	$updater->replaceEntryPoints();
	break;
	case 9:
	$updater->deleteOldFiles();
	break;
	case 10:
	$updater->moveNewVersionInPlace();
	break;
	case 11:
	$updater->setMaintenanceMode(false);
	break;
	case 12:
	$updater->finalize();
	break;
	}

Other considerations:

• maintenance mode / end-users hitting the server
• entrypoint replacement timing

[joshtrichards]

Related: #386