Speed up buildgraph loading by using some kind of daemon/service

This is an alternative approach for QBS-23.
What if QBS provide a daemon mode when a single instance of QBS starts and keeps working even when build process is finished. This instance can keep the buildgraph in memory and use it on demand. qbs commands will just add tasks to the running instance (build, install) which won't need to reload buildgraph for each task.
IDEs like Qt Creator could control this mode automatically what should speed up repeated builds especially when nothing in the project changed.