PersistedFiles should evict metadata for files that are no longer visible to query

[pauldix]

PersistedFiles has an add_snapshot method. When called this adds the metadata about Parquet files in object storage to this in memory structure. Now that we limit query look back to 72 hours, we should automatically evict any parquet metadata that is past this time range. We can just do it every time add_snapshot is called.