ZOOKEEPER-5011: Guard FileTxnSnapLog usage after close

[seekskyworld]

Issue: https://issues.apache.org/jira/browse/ZOOKEEPER-5011

Summary:

• add a closed-state guard so stale references throw an IllegalStateException instead of NPEs
• keep close idempotent and avoid marking the instance closed during truncateLog reopen
• add coverage for save-after-close and idempotent close

Motivation:

• improve diagnostics when stale ZooKeeperServer references hit FileTxnSnapLog after shutdown

Validation:

• maven3\current\bin\mvn.cmd -pl zookeeper-server -Dtest=FileTxnSnapLogTest test

[maoling]

To reproduce:

1. Application code obtains a reference to ZooKeeperServer (e.g., server = factory.getZooKeeperServer())
2. ...................................................................................................

Are you using an embedded ZooKeeperServer for in-JVM testing? If so, that might be why you're seeing this. This issue is typically not reproducible in a real-world cluster deployment?