internal/v4: add debug/pprof endpoint

This will make it possible to gain insight into how the server is behaving at runtime.

I've added a test for the endpoint that's most straightforward to test.
The others will need to be QA'd.

To QA, run the charmstore, then:

    go tool pprof http://$charmstorehost/v4/debug/pprof/heap

Could we put these behind the authentication? I'm not sure we want just anybody grabbing profile info.