Currently we have a lot of methods left in QueryShardContext that
take parsers or BytesReference arguments to do some xContent
parsing on the shard. While this still seems necessary in some cases
(e.g. percolation, phrase suggester), the shard context should only
be concerned with generating lucene queries from QueryBuilders.

This change removes all of the parseX() methods in favour of two
public methods toQuery(QueryBuilder) and toFilter(QueryBuilder) that
either call the query builders toFilter() or toQuery() method and
move all code required for parsing out to the respective callers.