Fixed race condition on deleting topic with active readers

[merlimat]

Motivation

When forcifully deleting a topic that has active readers, there is a race condition that lead to a failure in deleting the topic.
The race is

1. Connected readers are closed and removed
2. Since there's no active consumer on a non-durable subscription (eg: the reader sub), then we trigger the removal of the subscription and its cursor. That happens in background, in order to avoid deadlocking. See

   pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentSubscription.java

   Line 274 in 26c49a8

   topic.getBrokerService().pulsar().getExecutor().submit(() ->{

3. Once all consumers are closed, we then proceed in deleting the managed-ledger
4. To delete the managed-ledger, we get the current list of cursors and delete all of them in parallel
5. Since the cursors are removed in background, the list that we got might contain cursors that don't exist anymore when we try to delete them.

org.apache.bookkeeper.mledger.ManagedLedgerException: ManagedCursor not found: reader-7d300cd535
	at org.apache.pulsar.broker.service.persistent.PersistentTopic$4.deleteLedgerFailed(PersistentTopic.java:884) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl$16.deleteCursorFailed(ManagedLedgerImpl.java:2177) ~[org.apache.pulsar-managed-ledger-2.6.0.jar:2.6.0]
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.asyncDeleteCursor(ManagedLedgerImpl.java:766) ~[org.apache.pulsar-managed-ledger-2.6.0.jar:2.6.0]
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.asyncDelete(ManagedLedgerImpl.java:2156) ~[org.apache.pulsar-managed-ledger-2.6.0.jar:2.6.0]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.lambda$null$19(PersistentTopic.java:859) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
	at java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:859) ~[?:?]
	at java.util.concurrent.CompletableFuture.uniWhenCompleteStage(CompletableFuture.java:883) ~[?:?]
	at java.util.concurrent.CompletableFuture.whenComplete(CompletableFuture.java:2251) ~[?:?]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.lambda$delete$20(PersistentTopic.java:853) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
	at java.util.concurrent.CompletableFuture.uniAcceptNow(CompletableFuture.java:753) ~[?:?]
	at java.util.concurrent.CompletableFuture.uniAcceptStage(CompletableFuture.java:731) ~[?:?]
	at java.util.concurrent.CompletableFuture.thenAccept(CompletableFuture.java:2108) ~[?:?]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.delete(PersistentTopic.java:843) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.deleteForcefully(PersistentTopic.java:791) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
	at org.apache.pulsar.broker.service.BrokerService.deleteTopic(BrokerService.java:719) ~[org.apache.pulsar-pulsar-broker-2.6.0.jar:2.6.0]
Caused by: org.apache.bookkeeper.mledger.ManagedLedgerException: ManagedCursor not found: reader-7d300cd535

On topic deletion, we should be more tolerant of disappearing cursors and instead proceed as a success case.