Start 2.1.0 development.

Change-Id: Iebf7537c86629ce26c447b68b5e47814da15d456
Reviewed-on: http://review.couchbase.org/44011
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>