Bookie shutdown hook should trigger LedgerCacheImpl's indexPageManager flusher

**BUG REPORT**

1. Please describe the issue you observed:

As part of Bookie shutdown hook, InterleavedLedgerStorage.shutdown is called, which calls “entryLogger.shutdown()” and it flushes complete currentlogs and rotatedlogs. But in InterleavedLedgerStorage.shutdown for ledgercache, it calls “ledgerCache.close()” and it just flushes FileInfos but not in memory LedgerEntryPage instances. To be in sync with EntryLog files, indexPageManager (memory pages) should also be flushed.

https://github.com/apache/bookkeeper/blob/master/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/InterleavedLedgerStorage.java#L188

Start in bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/InterleavedLedgerStorage.java at the shutdown path described in the issue. Trace how entryLogger.shutdown(), ledgerCache.close(), and LedgerCacheImpl's indexPageManager handle pending data. Done means shutdown flushes in-memory LedgerEntryPage instances so they are consistent with the EntryLog files.

Written by the indexing model from the issue text.