NCBC-504: Adds support for VBucketForwardMaps

This commit adds a method for using VBucketForwardMaps. A later commit will
provide an implementation that uses the method.

Change-Id: I63607a3ab1148b7d4d8c3bf4fbd042c09e2aa978
Reviewed-on: http://review.couchbase.org/39854
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>