JCBC-647: Avoid WARN level log on failed over replica with observe.

Motivation
----------
During a failover/error condition, the actual number
of replicas will differ from the configured ones. There has always
been a check in place to not send a request to an inactive replica
(identified by -1 in the config). The code path used printed out
a WARN which, under high traffic, can seriously crowd the logs.

Modifications
-------------
The Locator already provides a pre-check to only return the active
replicas, which is used by replica get queries. This change also
makes the code use this path, only looping through the active
replicas and avoiding the very verbose WARN log message over
and over again.

Result
------
Less verbose logs, making it easier to discover actual problems.

Change-Id: Ie9577fd55f6171406a0176c8b7ec7a4ee4db4bcc
Reviewed-on: http://review.couchbase.org/44141
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>