Modernize SnapshotDeletionPolicy and PersistentSnapshotDeletionPolicy

[gsmiller]

Description

I bundled some modernization/cleanup changes here as I saw opportunities while going through this code for another project I'm working on. I think it's worth a modernization pass on this code (much of it is 13 years old). Also (semi-selfishly), since I've been away from Lucene for a few months, I'm using this as an opportunity to get my hands dirty again.

This change includes:

1. Small modernizations to be more idiomatic with newer versions of Java
2. Marking more methods synchronized (e.g., PersistentSnapshotDeletionPolicy#getLastSaveFile could technically have a race condition).
3. Adding consistent error recovery logic to PersistentSnapshotDeletionPolicy#release(long)
4. Tightening up the logic in PersistentSnapshotDeletionPolicy#loadPriorSnapshots to only read from the latest ref-count file (and to ensure all older files are cleaned up, not relying on the order the filenames are read from the directory).