Drush service clean-up
Drush service clean-up

* Move to class properties instead of static variables for caches
* Cache the Drush executable (in a class property)
* Fix aliases cache property, to run "drush sa" less often

pjcdawkins/platformsh-cli,pjcdawkins/platformsh-cli,pjcdawkins/platformsh-cli,platformsh/platformsh-cli,platformsh/platformsh-cli,platformsh/platformsh-cli