This commit notifies all users that are in a MUC on the local server node of
a shut down of the service. The notification conforms to XEP-0045.

In order to prevent a prolonged shut down, this commit:
- ensures that a shutdown routine is executed just once (instead of for each
  type of interface being shut down: module,component and listener)
- the presence stanza that is sent is constructed using in-memory objects
  only (no database lookups)
- all notification occurs in a thread pool, which will concurrently process
  them, but will terminate forcibly after a certain amount of time.