Handler refactor to centralize logging and error handling (google#144)

* Add infrastructure, convert sth handler

* Convert get-sth-consistency handler

* Convert the get-roots handler

and remove the rpc client param because it never makes any

* Convert the get proof by hash handler

* Convert get entries handler

* Convert get entry and proof handler

* Convert the add chain and pre chain handlers

* Remove the temporary .AsHandleFunc() stuff

* Review fixes.

Author: Martin2112