Adding a Lucene ft-index attribute boosting feature.

* Adding attribute boosting index configuration elements has-attr, has-sibling-attr, match-attr, match-sibling-attr as Lucene text element children, e g:

```xml
<text qname="@val">
  <match-sibling-attribute qname="att" value="writtenForm" boost="30"/>
  <match-sibling-attribute qname="att" value="definition" boost="15"/>
</text>
```
to be able to boost LMFish feature attribute/value attributes otherwise use default boost.
* All tests pass.
* Added the attribute boosting text element children to collection.xconf.xsd.