This fixes few cases where driver eventor goroutines are leaked during
normal operations, but especially so in tests.

This change makes few modifications:

First, it switches drivers to use `Context`s to manage shutdown events.
Previously, it relied on callers invoking `.Shutdown()` function that is
specific to internal drivers only and require casting.  Using `Contexts`
provide a consistent idiomatic way to manage lifecycle for both internal
and external drivers.

Also, I discovered few places where we don't clean up a temporary driver
instance in the plugin catalog code, where we dispense a driver to
inspect and validate the schema config without properly cleaning it up.