Make HEAD2HEAD updates work

Problem

• update_prepare_d8_bootstrap() performs a range of adjustments, which are only executed when the System module schema is < 8000, which means they are not executed for D8 sites.

Goal

• Allow me to upgrade my D8 DUG site. :)

Proposed solution

1. Use individual conditions for the bootstrap adjustments performed in update_prepare_d8_bootstrap().
2. Fix _drupal_bootstrap_configuration() to only set the error/exception handlers after config() is ready.
3. Fix DrupalKernel to account for a potentially empty module list.
4. Add update_get_schema_version() to retrieve the schema version of an extension.

Notes

• With attached patch, I'm successfully able to update from a ~6 months old database dump.
• Also attaching my head2head.php script, which needs to perform additional database adjustments before update.php is executed due to a few re-purposed module updates.

Related issues

• #1872522: Compiled data in PHP storage is cleared too late in drupal_flush_all_caches()
• #1881176: Revert bogus User module update number changes of Blocks to Plugins conversion
• #1333898: Support unstable2unstable upgrades? [policy, no patch]