For batch queries we have N different query_type's for one query, so
mapping a single query_type to a sample_provider doesn't really work
out.  Instead add a new constructor to construct a query directly
from a sample_provider.

Also, the sample buffer size needs to be determined at runtime, as
it depends on the number of query_types.

Signed-off-by: Rob Clark <robdclark@gmail.com>