Fix a problem with static segments being opened with an empty offset after switch unless active segments offset file was closed

Patch by Alex Petrov; reviewed by Aleksey Yeschenko for CASSANDRA-19867

src/java/org/apache/cassandra/journal/ActiveSegment.java

@@ -183,6 +183,7 @@ final class ActiveSegment<K, V> extends Segment<K, V>
     {
         index.persist(descriptor);
         metadata.persist(descriptor);
+        syncedOffsets.fsync();
         SyncUtil.trySyncDir(descriptor.directory);
     }
 

src/java/org/apache/cassandra/journal/SyncedOffsets.java

@@ -135,6 +135,7 @@ interface SyncedOffsets extends Closeable
             {
                 output.writeInt(offset);
                 output.writeInt((int) crc.getValue());
+                output.flush();
             }
             catch (IOException e)
             {