Reviewer Mike - Implement sproutlet tsx helper

This review is built on top of the branch with the SproutletAppServer shim code so you might want to review and merge that first before reviewing this (so github shows you just the new changes).  For your integration testing tomorrow, the file you care about is `sproutletproxy.cpp` and `sproutletproxy.h`.  You'll need to fill in the `TODO` in `create_response` to complete the last bit of work.

This code depends on https://github.com/Metaswitch/clearwater-app-servers/pull/5.

No UTs yet, but everything builds and existing UTs run.
Is this right?  The AppServer API and Sproutlet API just use int for fork_id, so why is this different?