SoftIndexFileStore does not support purge or expiration events

The SoftIndexFileStore will not returned expired entries on load or publish, however it does not support purge which in turn means no expiration events are generated from expired entries in the store.

We should do this by compacting the files. However, to reduce the amount of churn this generates each data file should know what the next time an entry may expire in it. This way we won't compact a file that doesn't have any entries that aren't yet expired unless it has enough free memory as normal.

When the normal compactor runs we cannot have it expire entries either as it must be done by the purge method so it can generate proper expiration events.