ZOOKEEPER-5055: Ensure FileTxnLog.close() closes every stream

[iamsanjaymalakar]

Problem

org.apache.zookeeper.server.persistence.FileTxnLog.close() returned immediately when closing logStream or an element of streamsToFlush threw java.io.IOException. Streams later in the cleanup sequence were therefore never closed. This can leak file descriptors during log shutdown.

Change

• Attempt every stream close.
• Retain the first java.io.IOException.
• Attach later close failures as suppressed exceptions.
• Throw the retained exception after cleanup completes.

Testing

• mvn -pl zookeeper-server -Dtest=FileTxnLogTest test