Wakeup the Selector on node add.

This can lead to an issue when no traffic is going through the
system that a newly added node does not get properly included
in the cluster landscape on the client side.

This has no effect if there is already going traffic through
the system.

Change-Id: Iebcf70bb7b3c093985b6fa4f6056a4153937b8cd
Reviewed-on: http://review.couchbase.org/25722
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>