Add rethinkdb.db_config pseudo-table for renaming databases.
Add unit test for rethinkdb.db_config pseudo-table.
Update comments in backend.hpp.
Update comments in backend.hpp. (2)
Merge branch 'tim_db_config' into reql_admin

Conflicts:
	src/clustering/administration/artificial_reql_cluster_interface.cc
	src/clustering/administration/artificial_reql_cluster_interface.hpp
	test/full_test/interface.test