nsqd: add /debug/pprof/{cmdline,symbol}

with these two endpoints you can directly profile a running nsqd:

    $ go tool pprof http://127.0.0.1:4151/debug/pprof/heap
    $ go tool pprof http://127.0.0.1:4151/debug/pprof/cpu
Merge pull request #248 from mreiferson/symbol_248

nsqd: support /debug/pprof/{cmdline,symbol}