[fix] [ml] Topics stats shows msgBacklog but there reality no backlog (apache#19275)

### Motivation

#### 1. `readPosition` point to a deleted ledger

When `trim ledgers` and `create new cursor` are executed concurrently, it will cause the `readPosition` of the cursor to point to a deleted ledger.

| time | `trim ledgers` | `create new cursor` |
| --- | --- | --- |
| 1 | | set read position and mark deleted position |
| 2 | delete ledger | |
| 3 | | add the cursor to `ManagedLedger.cursors` |

----

#### 2. Backlog wrong caused by `readPosition` wrong
<strong>(Highlight)</strong>Since the read position of the cursor is pointing at a deleted ledger, so deleted messages will never be consumed or acknowledged. Since the backlog in the API `topics stats` response is calculated as this: `managedLedger.entriesAddedCounter - cursor.messagesConsumedCounter`, the result is: Topics stats show `msgBacklog` but there is reality no backlog.
- `managedLedger.entriesAddedCounter`: Pulsar will set it to `0` when creating a new managed ledger, it will increment when adding entries.
- `cursor.messagesConsumedCounter`: Pulsar will set it to `0` when creating a new cursor, it will increment when acknowledging.

For example: 
- write entries to the managed ledger: `{1:0~1:9}...{5:0~5:9}`
  - `managedLedger.entriesAddedCounter` is `50` now
- create a new cursor, and set the read position to `1:0`
  -  `cursor.messagesConsumedCounter` is `0` now
- delete ledgers `1~4`
- consume all messages
  - can only consume the messages {5:0~5:9}, so `cursor.messagesConsumedCounter` is `10` now
- the `backlog` in response of `topics stats` is `50 - 10 = 40`, but there reality no backlog

----

#### 3. Reproduce issue
Sorry, I spent 4 hours trying to write a non-invasive test, but failed. <strong>(Highlight)</strong>You can reproduce by `testBacklogIfCursorCreateConcurrentWithTrimLedger` in the PR apache#19274

https://github.com/apache/pulsar/blob/a2cdc759fc2710e4dd913eb0485d23ebcaa076a4/pulsar-broker/src/test/java/org/apache/bookkeeper/mledger/impl/StatsBackLogTest.java#L163

### Modifications

Avoid the race condition of `cursor.initializeCursorPosition` and `internalTrimLedgers`

Author: poorbarcode

managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerImpl.java

@@ -993,12 +993,11 @@ public synchronized void asyncOpenCursor(final String cursorName, final InitialP
             public void operationComplete() {
                 log.info("[{}] Opened new cursor: {}", name, cursor);
                 cursor.setActive();
-                // Update the ack position (ignoring entries that were written while the cursor was being created)
-                cursor.initializeCursorPosition(InitialPosition.Earliest == initialPosition
-                        ? getFirstPositionAndCounter()
-                        : getLastPositionAndCounter());
-
                 synchronized (ManagedLedgerImpl.this) {
+                    // Update the ack position (ignoring entries that were written while the cursor was being created)
+                    cursor.initializeCursorPosition(InitialPosition.Earliest == initialPosition
+                            ? getFirstPositionAndCounter()
+                            : getLastPositionAndCounter());
                     addCursor(cursor);
                     uninitializedCursors.remove(cursorName).complete(cursor);
                 }