[ISSUE #251] Fix: checkAbnormalFuture of slave will not work if the network is abnormal

Co-authored-by: dengzhiwen1 <dengzhiwen1@xiaomi.com>

Author: cserwen

dledger/src/main/java/io/openmessaging/storage/dledger/DLedgerEntryPusher.java

@@ -1010,7 +1010,7 @@ private void checkAppendFuture(long endIndex) {
             if (minFastForwardIndex == Long.MAX_VALUE) {
                 return;
             }
-            Pair<PushEntryRequest, CompletableFuture<PushEntryResponse>> pair = writeRequestMap.get(minFastForwardIndex);
+            Pair<PushEntryRequest, CompletableFuture<PushEntryResponse>> pair = writeRequestMap.remove(minFastForwardIndex);
             if (pair == null) {
                 return;
             }