Query DSL: prefix query to support _id

[kimchy]

Even though we don't index the _id field explicitly, we can support prefix style query/filter on it (in a similar manner that we support term(s) query/filter on it - though there is also an explicit ids query).

There isn't really a need to add an ids_prefix style query/filter, since we can provide the same functionality automatically with prefix (again, in a similar manner that ids query/filter is no longer really needed).

The prefix query/filter will automatically work only against the specific type(s) if searching against the type (defined in the search request), or the field is prefixed by it (my_type._id as the prefix field name).