MB-7629, MB-7656 Delete context object

On context destruction, delete as well the context
object, and not just its members.

Change-Id: I25f3fb23d25e242fced7aa2d33d773c896509a12
Reviewed-on: http://review.couchbase.org/25307
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>