Skip sync the RocksDB when no changes (apache#3904)

Co-authored-by: Matteo Merli <mmerli@apache.org>
### Motivation
For the `LedgerMetadataIndex#removeDeletedLedgers` and `LedgerMetadataIndex#flush`, it will call ledgersDB sync whether the ledgersDB has changed or not. We can skip the sync call when nothing changed in the ledgersDB.

### Changes
- Check whether pendingLedgersUpdates is empty in `flush()` and `pendingDeletedLedgers` is empty in removeDeletedLedgers
- Move the `key.recycle()` in finally to cover keys leak when the ledgersDB operations throw an exception.

(cherry picked from commit 24464ba)

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/LedgerMetadataIndex.java

@@ -297,6 +297,10 @@ public void setMasterKey(long ledgerId, byte[] masterKey) throws IOException {
      * Flushes all pending changes.
      */
     public void flush() throws IOException {
+        if (pendingLedgersUpdates.isEmpty()) {
+            return;
+        }
+
         LongWrapper key = LongWrapper.get();
 
         try {
@@ -320,6 +324,10 @@ public void flush() throws IOException {
     }
 
     public void removeDeletedLedgers() throws IOException {
+        if (pendingDeletedLedgers.isEmpty()) {
+            return;
+        }
+
         LongWrapper key = LongWrapper.get();
 
         try {