* Add AsyncMigrationBase, update base classes and call async methods * Restored and obsoleted synchronous execution on IMigrationPlanExecutor. * Resolved breaking changes. * Fixed build. * Further obsoletes. * Fix build against v16/dev. * Removed and obsolete code related to post-migrations. * Removed service registration of unused interface. --------- Co-authored-by: Andy Butland <abutland73@gmail.com>