Ensure multi-shard temp. tables dropped on commit

We were relying on the default behavior, which would leave many orphan
tables during long-lived sessions. By specifying `ON COMMIT DROP` for
the temporary tables, we can ensure they are dropped as soon as the
query completes and no later.
Test temp. tables dropped immediately after query

The temporary tables we use for multi-shard queries should not persist
after the end of those queries, even if the session is still active.
Merge pull request #39 from citusdata/feature-drop_query_temp_tables-#32

Add ON COMMIT behavior to multi-shard temp. tables

cr: @sumedhpathak