Fallback to 'id' if user specified id field not present for explains

[softwaredoug]

This is based on this issue in Quepid.

In Splainer or Quepid, user specifies a custom id field (ie a field that's not "id") as the id. When this field is not acually Solr's unique key, we don't get explain information.

There's not a perfect solution. Proposed solution is to fallback to an 'id' field if present.