pimd: Ensure DR election happens when both sides change prio

Suppose we have 2 routers A and B.  Both Router A and B have
the same priority of 1000.  Router A is the elected DR.
Now suppose B lowers his priority to 1.  He still looses the
DR election and we are not sending a hello with the new priority.
Immediately after this A's priority is also lowered to 1, it
looses the election and sends the hello.  B receives this hello
and elects A as the DR( since it has the better ip address)
At this point A believes B is the DR, and B believes A is the
DR until such time that the normal hello from B is sent to A,
which if timed correctly can be a significant amount of time).

This code just causes a hello to be sent if the priority is
changed.  Now both sides will be able to converge quickly

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

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