Add Tuple* toString methods for better debugability.

Change-Id: I3e1b833295884cbfe79b5c94fca12fd91463dfcb
Reviewed-on: http://review.couchbase.org/44762
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>