Use new lease user contexts in RADIUS accounting
Migrated from https://oldkea.isc.org/ticket/5658
Current code has many potential problems and was scheduled to use use contexts from the beginning but it was postponed because user contexts in leases were implemented later:
- save/load to a CSV file is implemented but never tested.
- eraseCreateTimestamp() is called only when a STOP event is sent so the timestamp stays in memory without more control
- obviously using an user-context is the right way: extent following the lease one, save in stable storage, etc.
If memory leak on RADIUS accounting experiments are not conclusive this should be tried.
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