Implements a disk space thresholds feature, preventing the server from crashing or exiting of disks full.

Each instance of the monitor have two thresholds: "low" and "full". Other components can register their own handlers if they wish to receive notifications when the thresholds are reached. It extends the MonitorProvider interface and utilizes the update interval mechanism to poll the free disk space. The current free space and state of all monitor instances are exposed through the cn=monitor interface under the cn=Disk Space Monitor branch. The only component using the monitor now is the JE backend. When the "low" threshold is reached, write operations will only be permitted to users with the BYPASS_LOCKDOWN privilege. When the "full" threshold is reached, the backend is placed in read-only mode. The default "low" and "full" thresholds are 100 MB and 10MB respectively. Along with the new feature, this patch also added two privileges: SERVER_LOCKDOWN and BYPASS_LOCKDOWN. Any user with the SERVER_LOCKDOWN privilege can put and take the server out of lockdown mode. Any user with the BYPASS_LOCKDOWN mode can bypass lockdown mode. This was restricted to only "root" users before. In addition, the MonitorProvider abstract class no longer extends DirectoryThread. It now uses a static single threaded ScheduledExecutorService to update provider states. Any provider that wishes to have its state updated periodically needs to register its updater runnable with the scheduleUpdate method.