Change uniqueKey to use docValues and not stored field

This issue is about the performance improvements you can get by avoiding decompression during the first phase of a distributed search where only id and score is needed.

The improvements will be noticed for users if the docs are large or have lots of fields in them.
For users who don't have this scenario it shouldn't slow things done by any noticeable amounts?

We should default the unique key field to use docValuues='true' and stored='false'

Links to the discussion that lead to this idea:

• https://issues.apache.org/jira/browse/SOLR-5478?focusedCommentId=16036951&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16036951
• http://mail-archives.apache.org/mod_mbox/lucene-dev/201706.mbox/%3C008201d2ddf9%2429435740%247bca05c0%24%40thetaphi.de%3E