Convert common terms in query string to OR

So far this is just a test for what I expect to work when I finish:
Add support for converting the boolean clauses built by the query_string
query for chunks of query without any special syntax into common_terms
queries.  These should keep the query string's default operator.

Also add a configuration to the query string to switch the operator for
the low frequency terms in generated common terms queries to OR even if
the default query operator is AND.

Closes #4839