JCBC-261: Fix memcache compatibility on warmup backoff.

Since vbuckets are not supported and warmup is not needed on
memcache buckets, immediately get out of the loop and make sure
no vbucket access is done.

Change-Id: I3c48e37428a83ff8b0841614271622e3345d263a
Reviewed-on: http://review.couchbase.org/25083
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>