public synchronized long getLastUsedTimestamp() {
		return lastUsedTS;
	}
Called from the {@link GFileSystem#close()} before any destructive changes have
been made to gracefully shutdown the ref manager.
<p>
Broadcasts {@link FileSystemEventListener#onFilesystemClose(GFileSystem)}.
Ghidra/Features/Base/src/main/java/ghidra/formats/gfilesystem/FileSystemRefManager.java