drivers: Implement InternalPluginDriver interface

This implements the InternalPluginDriver interface in each driver, and
calls the cancellation fn for their respective eventers.

This fixes a per task goroutine leak during test suite execution.

drivers/java/driver.go

@@ -556,3 +556,7 @@ func GetAbsolutePath(bin string) (string, error) {
 
 	return filepath.EvalSymlinks(lp)
 }
+
+func (d *Driver) Shutdown() {
+	d.signalShutdown()
+}