Use unique thread name for group WALs

The consumer threads for each WAL group has the same name, since they only use the WAL root dir in the thread name.

new ThreadFactoryBuilder().setNameFormat("AsyncFSWAL-%d-" + rootDir.toString()).
  setDaemon(true).build()); 

For example, for BoundedGroupingStrategy, the consumer threads names are as follows,

We can use the log prefix instead, the consumer threads names will be changed to

So we can clearly see what happens from the log and the jstack info if something wrong with the WAL.