This commit sign the end of the "[WIP] allow to switch between trees" works.
The tree directory was used with s6-rc. Tree are now completely handled by
resolve file. That's means that services are not independent of a tree.
Again earn of flexibility.

Thanks for your service! This resolve file was used with s6-rc. The things now are simplier
and flexible.

service Master resolve file is no longer a service incorporate to the DB. It was reorganize to provide mandatory field to be a real Master. This file will now be used to known the global state of the services.