cleanup driver eventor goroutines

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.

client/pluginmanager/drivermanager/instance.go

@@ -248,10 +248,6 @@ func (i *instanceManager) cleanup() {
 		return
 	}
 
-	if internalPlugin, ok := i.plugin.Plugin().(drivers.InternalDriverPlugin); ok {
-		internalPlugin.Shutdown()
-	}
-
 	if !i.plugin.Exited() {
 		i.plugin.Kill()
 		if err := i.storeReattach(nil); err != nil {