Enable workhorse Go profiler server by default

See https://golang.org/pkg/net/http/pprof/#pkg-overview

On top of StackDriver continuous profiling, which is baked into LabKit, Workhorse has support for starting a pprof server that one can connect to over TCP. It exposes several endpoints to pull CPU, heap etc profiles. The overhead is minimal since it's lazy; you only pay the cost for importing this handler by default. CPU profiles would still have to be triggered by visiting a specific URL (see docs)

I think this should be enabled by default.

Note that I decided to merely expose the profiler port, not bind it to the host machine. This is to not accidentally clash with something that's already running on 6060. You will therefore have to add an override that also binds 6060 to the host machine.