long newOplogEntryId() {
    return oplogEntryId.incrementAndGet();
  }
Creates and returns a new oplogEntryId for the given key. An oplogEntryId is needed when
storing a key/value pair on disk. A new one is only needed if the key is new. Otherwise the
oplogEntryId already allocated for a key can be reused for the same key.

@return A disk id that can be used to access this key/value pair on disk
geode-core/src/main/java/org/apache/geode/internal/cache/PersistentOplogSet.java