Added a technical task to the TODO list.
UnidirectionalUpgrader: implemented.
Upgrader: renamed.
ReflectiveUpgrader: replaces the callback upgrader.
Now using the reflective upgrader in Chrony's SQLite event repository.
Extracted the SQL schema management from the event repository.
SQLiteEventRepositorySchema: a bit of cleanup.
ReflectiveUpgrader: some code sanitizing.
UpgradeScript: removed.