Fix return type error in Startspan

Merged to master.

Minor comment: I suggest using emplace_back instead of construct+push_back.

@Paul Price, since I've overwritten all of your changes, I'm just sending it back to you for review.

Jenkins run is underway at https://ci.lsst.codes/blue/organizations/jenkins/stack-os-matrix/detail/stack-os-matrix/27914/pipeline.

Good news is that all the really bad stuff was in code that was completely unused (templates only instantiated in blocks that are already #ifdef'd out). I've just removed that.

There are a few more things I'm still cleaning up in that file in code that is in active use. All that code is correct, so far, but I'm sure at least some of it emits warnings, and there's some very overzealous use of shared_ptr I'd like to trim down for efficiency reasons.

I messed up the Jenkins run (specified wrong branch), and @Jim Bosch wants to contribute some more.