Allow `artificial_table_backend_t` to return a stream or vector of rows
Fixes the artificial tables to use the new interface
Fixes review feedback
Updates the unit tests to use the new interface
Merge branch 'reql_admin' into tim/2980

Conflicts:
	src/clustering/administration/tables/table_status.cc
Use throttled_pmap() instead of a loop for reading rows of table_status.