query-parallel-or: always use sub ctx to create temporary tables
query-parallel-or: always use sub ctx to create temporary tables

We need to use sub ctx for creating temporary tables in parallel. If
we use parent ctx for creating temporary tables, lock isn't acquired.
We use a sub ctx, lock is acquired for all sub ctxs. If we use a
parent ctx, lock isn't acquired.

groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga