SubscriptionManager.run(): Honor InterruptedException

- sendNotifications() has recently been changed to throw it.
- It is a shutdown request, and this thread is a loop which iterates
over a todo-list of separate stuff (Clients in this case), so its the
perfect place to deal with InterruptedException by returning early.
- Notice that the remainder catch block can be RuntimeException instead
of Exception since the functions called in the try{} have no further
throws declarations than InterruptedException.
SubscriptionManager: Add performance TODO

Currently, all notifications are deployed one by one in a single
thread which loops over all clients. This causes each client to block
deployment for all of the other clients. A better concept would be to
have one thread per client.
Notice that this TODO is also enforced by the bugtracker entry
https://bugs.freenetproject.org/view.php?id=6350 which requests the
developers to search the source code for "TODO: Optimization" soon
enough.
SubscriptionManager: Remove FIXME of handling InterruptedException

Was resolved by the previous commits.