net/announce: Add optional ID

Previously there was a single instance of the timer used by
monitor triggered announces, that's OK, but when combined with the
previous change that lets you have announces for subsets of interfaces
it's a bit restrictive if you want to do different things to different
interfaces.

Add an 'id' field to the announce, and maintain a list of the
timers based on id.

This allows you to for example:
    a) Start an announce going on interface eth0 for a long time
    b) Start an announce going on interface eth1 for a long time
    c) Kill the announce on eth0 while leaving eth1 going.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>