Parallelize creation of vbuckets during warmup

Right now we create all vbuckets during warmup from the same
thread. We also call the initVBuckets function multiple times
during wamrup and although we will only create the vbuckets once
calling this function over and over again is unnecessary. This
change introduces a new warmup state for creating vbuckets.

Change-Id: If0b49d618408f646492168c39a59d7aa34b07b50
Reviewed-on: http://review.couchbase.org/32894
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>