MB-11999: Preferential loading of active/replica vbuckets

During warmup, order the vbucket list such that
active vbuckets get 60% preference while replica
vbuckets get 40% preference.

Example:
In a 4 node cluster (DGM), approx. resident ratios:
1. Before warmup
    active:     36%     34%     34%     41%
    replica:    33%     35%     35%     27%
2. After warmup
    active:     40%     42%     41%     46%
    replica:    31%     29%     29%     24%

Change-Id: I60e0427bca58530247086d730135ebb4be70bb84
Reviewed-on: http://review.couchbase.org/41468
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>