Fill in artificial_table_t stub methods. This commit compiles but has not been tested.
Fix a bug. Update arraylimits test to reflect new behavior on inserting a too-large value into a table.
Add way to test artificial tables

Adds a new artificial table "rethinkdb._debug_scratch" which stores
arbitrary data in memory. In theory a test could use this to
exercise the artificial_table_t class.
Fix a bug in in_memory_artificial_table_backend_t
Fix a crash in read_all_primary_keys() for table_config and table_status pseudo-tables.
Fix some style errors.
Merge branch 'reql_admin' into tim_real_artificial_table
Clarify in_memory_artificial_table_backend_t::block()
Pass counted_t<const ql::datum_t> by const reference for slightly better performance.
Add a comment explaining why we don't need std::stable_sort.
Use the user-specified configured_limits_t when constructing a stats response.
Dont put a hyphen in the word changefeeds.
Add a note explaining what exceptions do_single_update() can throw.
Merge branch 'tim_real_artificial_table' into reql_admin