msgr: adjust nonce on rebind()

We can have a situation where:

 - we have a pipe to a peer
 - pipe goes to standby (on peer)
 - we rebind to a new port
 - ....
 - we rebind again to the same old port
 - we connect to peer

and get reattached to the ancient pipe from two instances back.  Avoid that
by picking a new nonce each time we rebind.

Add 1,000,000 each time so that the port is still legible in the printed
output.

Signed-off-by: Sage Weil <sage@inktank.com>