rgw/datalog: Manage and shutdown tasks properly

rgw/datalog: Manage and shutdown tasks properly

This is slightly ugly but good enough for now. Make sure we can block
when shutting down background tasks.

Remove a few `driver` parameters that are unused. This lets us
simplify the IAM Policy and Lua tests and not construct stores we
never use. (Which is good since we aren't running them under a cluster.)

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>