Change interface.specific_table test so that it tests artificial tables and rename it to interface.artificial_table. Also, fix a test error.
Make artificial_table_t use a special datum_stream_t subclass that fetches results incrementally.

Now that it no longer uses array_datum_stream_t, it won't appear as an array when users call .type_of(), so the interface.artificial_table test passes.
Merge branch 'reql_admin' into tim_test_artificial_table

Conflicts:
	src/rdb_protocol/artificial_table/artificial_table.cc
	test/full_test/interface.test