ISSUE-14268: [2.8.1] FGC and throw NPE

[sijie]

Original Issue: apache#14268

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Describe the bug
The phenomenon of our online problem is as follows:
1. NPE exception: (This is the same as the NPE exception in your PR, but only this type of NPE was found. I see two NPEs mentioned in your PR)
03:22:50.442 [BookKeeperClientWorker-OrderedExecutor-4-0] ERROR org.apache.bookkeeper.common.util.SafeRunnable - Unexpected throwable caught
java.lang.NullPointerException: null
at org.apache.bookkeeper.mledger.impl.OpAddEntry.addComplete(OpAddEntry.java:153) ~[org.apache.pulsar-managed-ledger-2.8.1.2.jar:2.8.1.2]
at
2. Frequent FGC, I dumped the memory and found that there are a lot of pendingAddOp objects (more than seven million), but the write traffic has dropped to the bottom:

3.Looking at the log, some thread pools take too long to execute tasks, even reaching the hour level:
03:23:47.838 [bookkeeper-ml-scheduler-OrderedScheduler-54-0] WARN org.apache.bookkeeper.common.util.OrderedExecutor - Runnable org.apache.bookkeeper.mledger.impl.ManagedCursorImpl$$Lambda$1000/1597732433@2fc9a98e:class org.apache.bookkeeper.mledger.impl.ManagedCursorImpl$$Lambda$1000/1597732433 took too long 12721684 micros to execute.
03:24:53.508 [bookkeeper-ml-scheduler-OrderedScheduler-32-0] WARN org.apache.bookkeeper.common.util.OrderedExecutor - Runnable org.apache.bookkeeper.common.util.OrderedExecutor$TimedRunnable@3f387130:class org.apache.bookkeeper.common.util.OrderedExecutor$TimedRunnable took too long 16594876 micros to execute.
03:26:02.079 [bookkeeper-ml-scheduler-OrderedScheduler-35-0] WARN org.apache.bookkeeper.common.util.OrderedExecutor - Runnable org.apache.bookkeeper.mledger.impl.ManagedLedgerFactoryImpl$$Lambda$92/1773008684@4ddf5185:class org.apache.bookkeeper.mledger.impl.ManagedLedgerFactoryImpl$$Lambda$92/1773008684 took too long 17111879 micros to execute.

4. zookkepeer session timeout:
03:26:02.079 [bookkeeper-ml-scheduler-OrderedScheduler-35-0] WARN org.apache.bookkeeper.common.util.OrderedExecutor - Runnable org.apache.bookkeeper.mledger.impl.ManagedLedgerFactoryImpl$$Lambda$92/1773008684@4ddf5185:class org.apache.bookkeeper.mledger.impl.ManagedLedgerFactoryImpl$$Lambda$92/1773008684 took too long 17111879 micros to execute.

I noticed there is this PR that may be related to this issue: apache#12993