allow add on expired keys

[kmpm]

Closes #1993

This is very useful when using dedupe on fairly static keys which otherwise would only trigger compaction when new keys are added.

This change simulate compaction by checking if TTL expired on an existing key, much like the Get function.
If expired, but no compaction is done (because of compaction_interval) the value is only updated.

Since the Add operation technically is a write I don't see a reason to change the documentation The compaction interval determines how often the cache is cleared of expired items, and this process is only triggered on writes to the cache. Access to the cache is blocked during this process.