Ensure l3 agent receives notification about added router

Ensure l3 agent receives notification about added router

Currently router_added (and other) notifications are sent
to agents with an RPC cast() method which does not ensure that
the message is actually delivered to the recipient.
If the message is lost (due to instability of messaging system
in some failover scenarios for example) neither server nor agent
will be aware of that and router will be "lost" till next agent
resync. Resync will only happen in case of errors on agent side
or restart.
The fix makes server use call() to notify agents about added routers
thus ensuring no routers will be lost.

This also unifies reschedule_router() method to avoid code duplication
between legacy and dvr agent schedulers.

Closes-Bug: #1482630
Related-Bug #1404743
Change-Id: Id08764ba837d8f47a28649d081a5876797fe369e