Hierarchical paths for WALs

Currently all WALs are just dumped into a single directory irrespective of the ServerName for the regionserver to which they belong.
This is a problem specifically for the WALs archive directory (HConstants.HREGION_OLDLOGDIR_NAME) since it holds a large number of files which can easily make it go above the HDFS max items in a directory limit for large clusters in scenarios like pausing replication. HDFS imposes a hard limit of around 6 million files in a single directory due to Protobuf serialization limitations.

We should put WALs for a particular servername in its own directory inside the WALs (archive) directory.