Apply QueryParser boost to top leve query if applicable.

Set the query boost of a parsed query string query to the product of
the parsed query boost and the boost value specified in the "boost"
query string parameter. This only applies if the top level query returned
from the query parser has a boost assigned to it. In such a case we must
multiply the boost with the top level query boost otherwise the boost
will be overwritten ie. 'foo^2' has a top-level boost of 2 while
'foo^2 OR bar^3' has a top level boost of 1.0 (default) since the
boolean query is the top level query.

Closes #3024