* 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.
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Co-authored-by: Andy Butland <abutland73@gmail.com>
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