[Reviewer: Mike] Don't add 2 references to statelessly sent requests

Mike,

I think the previous fix (https://github.com/Metaswitch/sprout/commit/5ed33cbb80e658a71a16e0ab081d50ade71d7680) was just wrong - it added code to add an additional reference but didn't remove the old one.  My fix is not to add the second reference.  As well as avoiding a spurious reference, it also fixes #555, which covers the original problem - that `tdata` can already have been destroyed by the time we hit the success branch.

I've run the UT but not added any new test cases - I don't think we can easily UT this.

Matt