Use "qf" param for cleaner boosts and field limits

Before: q=(MyField:+term OR MyOtherField:+term^1.5)
After: q=term&qf=MyField MyOtherField^1.5

Also fixes a bug where boost on some fields only would limit
the actually searched fields (need to be mentioned explicitly)
Recipe for boosting records
Use SolrIndex->getIndexName() consistently

That's important in case we need to disambiguate the index,
e.g. when multiple copies of the same codebase run on the same
environment and server. Example implementation:

function getIndexName() {
	return preg_replace('/[^\w]/i', '-', __FILE__);
}