OPENDJ-1809 CR-6564 Disk Space Monitoring should be a server wide service

Disk space monitoring is now a global service, per DirectoryServer instance.
Backends can get the service through a serverContext and will be notified of any trigger of the configured thresholds.
Instead of using raw directories, if the JVM supports it, try to gather the mountpoints or global root folders and monitor those
instead. Still messages logged by the backends will reference only their own directories.
Each backend can also log its own messages in addition to what already logged by the general service.