JCBC-223: Do not broadcast op on observe to -1 replicas.

This changeset changes observe behavior so that no broadcast
operation is sent to -1 replicas. This prevents a possible
exception when using only one node in the cluster but replicas
are defined (but not available).

Change-Id: I1995610efc17fd60046eada7dfc94c459c885208
Reviewed-on: http://review.couchbase.org/24261
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Tugdual Grall <tugdual@gmail.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>