shardHandlerFactory is not properly persisted

In SOLR-5028 I discovered persistence for shardHandlerFactory is only looking for connTimeout and socketTimeout children. Persistence should work for any SHF impl, not just HttpShardHandlerFactory. I think the thing to do here is just copying the underlying Node to the new file, but the current persistence code assumes a flat (String->String) hierarchy (which seems wrong, flat hierarchy was one of the reasons myself and others were against solr.properties).