fix(net): stop BackupServer.close() hanging while bind is in flight

[halibobo1205]

What does this PR do?

Fix a BackupServer shutdown race that can cause BackupServerTest to time out.

Create the event loop group in initServer(), before the server task is submitted, so close() can shut it down while bind is still pending.

close() now interrupts the server executor and waits up to 10 seconds for the event loop group, replacing the ExecutorServiceManager.shutdownAndAwaitTermination call that could wait up to 120 seconds across two termination waits. The server task requests group shutdown in finally without blocking again.

Why are these changes required?

Previously, close() closed the channel only if it had already been published, then called ExecutorServiceManager.shutdownAndAwaitTermination. If bind completed after close() checked the channel, nothing closed the socket, the server thread stayed parked in closeFuture().sync(), and close() blocked in awaitTermination long enough to trip the test's 60-second timeout.

Related CI failure (attempt 2 of that workflow run; the test failed all six attempts—the initial run plus five retries), abbreviated:

org.tron.common.backup.BackupServerTest > test FAILED
    org.junit.runners.model.TestTimedOutException: test timed out after 60 seconds
        ...
        at java.util.concurrent.ThreadPoolExecutor.awaitTermination(ThreadPoolExecutor.java:1475)
        at java.util.concurrent.Executors$DelegatedExecutorService.awaitTermination(Executors.java:675)
        at org.tron.common.es.ExecutorServiceManager.shutdownAndAwaitTermination(ExecutorServiceManager.java:90)
        at org.tron.common.backup.socket.BackupServer.close(BackupServer.java:106)
        at org.tron.common.backup.BackupServerTest.tearDown(BackupServerTest.java:42)

This PR has been tested by:

• closeAfterStarted: waits for a successfully bound channel, closes the server, and verifies that the channel closes and both the group and executor terminate.
• closeWhileBindIsPending: deterministically holds bind registration pending, initiates shutdown, then releases the event loop and verifies termination.
• BackupManagerTest now uses backupServer.close() for cleanup.

[halibobo1205]

Fixed by #6961