MB-7435: Add tap-vbtakeover to cbstats

Change-Id: I77b03dee6ba4c76b713f1b68d61e9e079b1849e8
Reviewed-on: http://review.couchbase.org/25884
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>