Use a thread to provide the HTTP reception-side service

The events it will trigger will be handled in a new thread, letting the HTTP service handle the servlet in the current thread
=> avoids dead locks
Corrected *Group methods in Herald core service
Added support for directory group listeners

- group_set(group): called the first time a peer is seen in the group, i.e. when the group is created
- group_unset(group): called when the last peer of the group goes away, i.e. when the group is deleted
Commit of the missing interface IDirectoryGroupListener :/
EventData.getError(): Avoid a NullPointerException
Event: call Condition.signalAll(), not notifyAll() :/