when setExplicitLac is called the ledgersCount should increased (#4138)

The **ledgersCount.incrementAndGet** should be called in **setExplicitLac** , just like in **setFenced** and **setLimbo**.

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

@@ -403,6 +403,7 @@ void setExplicitLac(long ledgerId, ByteBuf lac) throws IOException {
 
             if (ledgers.put(ledgerId, newLedgerData) == null) {
                 // Ledger had been deleted
+                ledgersCount.incrementAndGet();
                 return;
             } else if (log.isDebugEnabled()) {
                 log.debug("Set explicitLac on ledger {}", ledgerId);